Simple make the class partial, and then use the same name in the two classes.
public partial class MyClass
{
// code....
}
The other class need to be equal:
public partial class MyClass
{
// code....
}
From MSDN:
"Partial type definitions allow for the definition of a class, struct, or interface to be split into multiple files
See more here: http://msdn.microsoft.com/en-us/library/wbx7zzdd.aspx