I was wondering if it's possible to make an app on android communicate with one on Windows Phone 7, or if there's even a way to develop apps for both platforms in the same language.
The best way would likely be communicating via a web server to pass the data too and from each device. It depends on what you intend to pass between them though.
Network sockets will allow communication between ANY devices.
It is possible to share some(!) code of the business layer logic.
There is a mono framework for android and iphone. You can use it to write code in C# for both devices.
However, the UI is device-dependent, so that cannot be shared.
A lot of quick and dirty ways now to develop apps for cross-platform is to use a mini-browser windows that simply points to a URL with no toolbars such as m.example.com.
Developing your "app" site in HTML5 allows a lot of possibilities to open up as far as what you can do from there. This gives you the opportunity to work a lot with cross-platform databases and dynamic data as well.