One thing to mention:
If you record the guitar straight DI (listening to the AmpSim plugin while tracking), you can always "re-amp" the straight DI signal out to the SansAmp and record its output to another track.
You can always flip the phase in software... so no need to worry about doing that via hardware.
 

Perhaps I have missed something here but you can also achieve this inside CbB by using Aux tracks or Patch points I believe.

1. Insert an Audio track
2. On the Output of this track Create a new Patch Point.
3. Insert two new Audio tracks
4. Set the input of each track to the Patch point created above
5. Put your preferred Amp sims one in each of the tracks FX Bins, say GR5 in one and TH 3 in the other.
6. Set the first track to input monitor the two patch point tracks will should already be set to input monitor
7. Arm for recording the two tracks with the Amp sims in and away you go.

This was as suggested by Jim above with patch points you can have an number of additional tracks fed from the one source.