I dont know whether it helps but,, well,, as long as it is a 2x2 MIMO channel then it means there exists 4 SISO channels. As long as you assume that they are independent of each other (due to lambda/2 antenna spacing) all you need to do is to create 4 SISO channel models independent of each other. If you are using Matlab, go to help file and read how to define a SISO "rayleigh" channel model with doppler and path delays... Well,, sorry this answer might have been a bit simple or useless if you may already know these.