Two lines (two numbers) on one mobile phone is a popular demand. All non-ancient GSM phones support multiple lines, but it seems that no carrier in the world bother to support it. This is true a couple of years ago to last two years, not very true now, but the biggest and most popular carriers don't bother. Now still no US carrier bothers. Now the phones are so smart that you can do a lot of things to by pass the carriers, forcing some carriers to compete. The smaller carriers might just as well provide some fringe benefits to increase head count.
For GSM and 3G(UMTS) you can play around with the SIM cards. For other carriers, you can call via some calling card like services, or Internet services.
You can buy a super SIM card, together with a card reader and writer. You copy all your carrier SIM cards over to your own super SIM card. I heard that 3G SIM's are not possible to copy due to encryption. The original SIMs have to be compatible to your super SIM and to your phone, which makes this the most problematic method, though the most elegant.
The most common method is to get a dual-SIM adapter. I think Magicsim is the most common brand on eBay and I had one, long before they have a brand name and a website online. SIM's have some sort of standards, including V1, V2 and 3G. There are also different memory sizes, 32K, 64K to 128K (3G). Make sure that your phone is on the adapter compatible list. I don' t know if it's possible that some carrier SIM's can cause further compatibility problems. But I don't think it will be if your phone is a 3G or late GSM model.
There are two types of dual-SIM adapters. The simpler type looks like three SIM cards connected together. There are two holders for your carrier SIM cards. You insert the third card into the SIM compartment of your phone. This type requires that your battery compartment isn't too tight to contain the extra thickness of the SIM cards. You are OK if your phone is on the compatibility list. I would recommend this though I had the other type.
The second dual-SIM adapter requires you to cut out the central bit of your SIM cards, and put them into a SIM card that holds your two little cut out pieces. The adapter in the form of a SIM card is slightly thicker than an ordinary SIM card, so it might not fit if your SIM compartment is super thin. Or the contact may be loose so they provide you with sponge fillers for a tight fit.
The cut out type adapters sound dangerous and difficult. It's yes or no. SIM cards don't cost much if you know where to find, and if you have a plan your carrier may replace it for you. Unless you make very stupid cuts, even if the adapter doesn't work, they provide two holders for you to hold your cards for use as if they are not cut. I advise to get a SIM cutter, which I didn't. I just used a scissor. I intended to cut a little larger than the template, then gradually reduce the size to fit the adapter. But I ended up simply chopped off the excess with one simple cut per straight edge.
The two cut out SIM cards should look exactly like two miniature SIM cards, but not mine. After a little trimming they can go inside the adapter. But with some loose space one of the miniature card can move a little inside. It didn't work at first, or it didn't work reliably. But once I swapped the two miniature cards, they fit better and never gave me problems. Maybe after a year the SIM's are lose again. So I pull out and plug in the cards again and added the sponge filler to hold the cards tighter. It works fine now.
I brought a 64K SIM served by AT&T (previously Cingular). It was another eBay order without phone and plan. It was for the GoPhone, prepaid or with a monthly plan. It might be possible that the carrier may not activate some cards, they way they are sold or according to their serial numbers. My SIMs are OK but probably because I wanted to port my old phone number to the prepaid phone(!), I went through several operators. Virgin had better service and better deal, but they are not GSM, reselling Sprint airtime.
I had a carrier unlocked GSM phone for traveling. I don't think you need an unlock phone when you are using two SIMs from the same carrier. If you ask, a nice carrier may unlock the phone for you after 6 months. But most phones are trivial to unlock yourself. For Moto you only need to download some software to the PC and a standard mini USB cable. I don't know about the latest 3G phones but you always search unlock hacks specific for your phone. If you travel abroad those Blade Runner (the movie) type shops will unlock it for you free if you buy anything trivial.
Adapters cost little but you pay for those guys who buy it in bulk and resell on eBay. Same for SIM cards. US carriers don't sell SIM cards directly without anything to go with it. But somehow you can get it on eBay. Abroad, SIM cards with airtime are cheaper than refilling the airtime itself.
At any one time, only one SIM and one number is active. In the older adapters, you switch off the phone and then back on to switch SIM cards, hence your number. In the newer ones, you dial 001 and then hangup to use SIM card number one, and so on. With better compatibility, you can pick SIM1 or SIM2 on your phone menu. You can also rotate the cards at fixed intervals, x minutes for the 1st and y minutes for the 2nd. This way, you can receive all your voicemail and other alerts without having to manually switch the SIM cards. But I don't feel comfortable doing this. My adapter doesn't work in this mode anyway.
Normally you set your phone to forward on unavailable to the other number, so you will not miss any calls. But there are minor problems. You don't know if the call is forwarded or not. You just have the caller ID. So if you do not know the person, you don't know which number the caller called. And be very careful if your numbers aren't public. Your line 1 calls will be forwarded to line 2, and voice mail will be left on your line 2 account. The system default message is to announce your line 2 phone number, thought the caller called your line 1 number. The same problem occurs again if line 1 is supposed to be for John and line 2 for Peter. On the voice mail greetings you have to swap the greetings for John and Peter. But when you sometimes turned off the call forward for some reasons, you have to swap the greetings. So carrying two phones isn't a bad idea.
The cheapest way to keep an extra number this way is prepaid, roughly $100 a year or $25 for 3 months from various carriers, if you call very little. It's difficult to compare exactly because most have some refill bonus. Most carrier will allow you to go offline a few months while keeping your number. It's much cheaper if your usage patterns allow for that. AT&T (Cingular) has 3G, T-mobile still haven't make the announcement. And of course AT&T has the iPhone. With the constant cat and mouse unlock the iPhone game, no way AT&T is going to unlock your iPhone for your travels.
By the way, there are phones holding two SIM cards. But I bet they aren't too good as the demand is perhaps 1 in 1,000 or 100,000. And then there's a version of Blackberry from Verzion, which provides CDMA2000 technology, but also duals as a GSM phone for you to travel. Of course the GSM phone is unlocked for foreign carriers. And since you already picked Verizon, they are not afraid that you want to switch to GSM/3G carriers. But if money is no object, there are other more elegant ways to have a 2nd phone number with a smart phone.
The Internet or calling card like methods will be next.