If you are using the latest software release (m257)
layer / mix specs are as follows:

PRO - 32 layers 16 mixes 12 video inputs, 10 DMX PixelMad output
DV - 8 layers 8 mixes 12 video inputs, 4 DMX PixelMad output
Express - 4 layers 6 mixes 12 video inputs, 4 DMX PixelMad output
Lite - 2 layers 4 mixes 12 video inputs, 1 DMX PixelMad output

If you have an EXPRESS licence - then your scenario with a TH2G should work as you have access to 4 layers and 6 mixes available....
This should all work on any intel MacPro from 2007 onwards.

What version of software are you using?

S