I got around the problem a little bit. I've not managed two tablets but:
I have one Wacom tablet using Osculator, which has the option to disconnect it from the mouse function, so you can simultaneously use the trackpad.
I'm also using Touch OSC on an iPad which has an xy-pad feature that works like a graphics tablet for the x,y axis at least. There's a little latency but you can do it via usb as well.
I've seen that it's possible to connect two computers, using Osculator on both. That way you could use a graphics tablet on that one and transfer the OSC data from one computer to the other. This sounds like the most promising method, though I've not managed to get it to work yet. I only just had a quick go though so I'm sure it might work.