Actually, you can.
Don't maximize Sonar but manually drag it to expand entirely over two monitors. Open both projects and make sure you have only the one screen open in each project (i.e. no extra plugs or undocked browsers etc). Then select "tile in columns" from the "window" menu.
What you can't do is move tracks between project (which is a shame, Ableton does this and it's incredibly useful), but you can move audio and midi clips back and forth quite easily. Plugins between bins too (and they will retain their settings), but not the regular ProChannel modules.