Of course you can, but not quite like you know it from Windows. You can't spread out the WB over two screens, but you can run Program A in screenmode sayCVision800x600 and Programm B in screenmode PicassoIV1024x768 at the same time.
Someone even wrote a little arrexscript that automatically gimes an [Amiga]+m when you reach the screenborder. Must be on Aminet