Can I use 2 different countries sim card in an Iphone?

I have an american contract (verizon) but live overseas and Id like to bring my Iphone with me to London and put in and Orange sim card. Can I do this?

Yes. You can do this. My dad has an iPhone too… He did the same thing when he went overseas.

Probably not. Verizon is a CDMA network which does not use SIM cards & Orange is a GSM network that does.
The phone would have to be unlocked in order for other SIM cards to work in it.

With Verizon, only the iPhone 4S and iPhone 5 are "global" phones which take SIM cards; the iPhone 4 and older don't. So you are out of luck if you don't have an iPhone 4S or 5.

Since Verizon is normally CDMA (not GSM which uses SIM cards), Verizon global phones are designed to only use GSM when they're outside the USA. Verizon actually includes a SIM card with all of their global phones, so that they'll work right away when you're outside of the USA (but you'll continue to be billed by Verizon, at higher rates).

If you want to put in a different carrier's SIM card, like from Orange, you can do that, but first have to get your phone "unlocked" (this isn't the same as jailbreaking). Verizon can tell you how to do this over the phone if you call their customer service. Once you've done that, your iPhone 4S or iPhone 5 will work with any carrier's SIM card (Note that Verizon will not unlock brand new phones; I think you have to have had your phone for at least several months first).

Remember to get the right sized SIM card - I think iPhone 4S takes "micro SIM" and 5 takes "nano SIM". Both of these are smaller than just plain regular SIM cards.