The Dumble-type 6L6 amps are mid heavy.  It's that smooth pronounced mid-range that distinguishes the Dumble cleans from blackface amps from which they were derived.  In the same way, the Trainwreck El-34 amps are distinct from Marshall-type amps because of their heavy mids.  Compare your Expression with a plexi... the voicing is different, but its the strong mids in the Trainwreck that really stand out.

Generally, the point of a multi-amp rig (as opposed to multiple amps run one at a time) it to put together amps that complement each other and fill in the others' weaknesses.  Bonamassa does this with a Dumble-style amp running with a Silver Jubilee.  The Silver Jubilee, like most Marshalls, has strong highs and lows and hollow mids.  The two amps together give a strong sound palette from lows to highs.

Combining a Dumble and Trainwreck would simply put two mid heavy amps together, likely covering up the sonic signatures of each.

The Trainwreck amps are bright with strong mids and somewhat weak lows.  Maybe something with a big botton (Diezel?) would fill it out.

I would like to someday try the Bonamassa thing.  When listening to him, the Dumble voice is largely covered by the Marshall sound.  Actually, I find his sound is comparable to a Trainwreck because of the strong mids.  My thought is to combine a OTS OD channel with a DC-30 EF channel run clean.  I think the chime of the DC30 would be an interesting complement with the OTS.

Absolutely.  Whether using two amps or two mics at different distances from the speaker, phase alignment is an issue.  The Radial Phazer is one tool.  Little Labs also makes a unit to align phase. 

For my applications, I like to add time-based effects after the amp.  So in a two-amp rig, the speaker outs go to a tool like the Palmer PDI-03.  From a pair of PDI-03s to the time-based effects.  Then any post-processing EQ and compression, then to phase alignment.