How do I make the Windows Remote Assistance window full screen accross two monitors?

Unfortunately Remote Assistance is not multi-screen aware and this is not possible.

At a stretch, you may be able to drag and make the application span across two screens but I am not sure that will work.

You can try looking in your graphics card driver to see if it supports a mode that basically allows your machine to act as one large screen instead of 2/3 smaller ones, however I think only Matrox display drivers can do that out the box.

Sorry I couldn't really be more help here.