There are a bunch of collaborative text editing web sites where two or more people can work on the same document. I've used them for technical phone interviews

There are a bunch of collaborative text editing web sites where two or more people can work on the same document. I've used them for technical phone interviews and they work well. Combine that with phone and skype and you can make things happen (of course it's hard to run code in this). Perhaps a screen sharing / webex approach would work in the case where you really need to run code together.