Gain is gain, it is constant no matter what the input voltage (until clipping, that is). So yes, in theory your hookup would work. The only caution has to do with tolerances, i.e., is the gain exactly 29.0 dB or is it actually 28.2 dB? Same goes for the other amp. You would need a high quality calibrated voltmeter to be certain.
Gain matching of power amps
How important is it to have amps of same gain when using amps from different manufacturers in a horizontal biamp? Currently using Audiolab 8000P's(29db gain) in vertical biamp configuration, but need to tame metal tweeter top end. Looking at McCormack(40db gain), and Classe power amps(anyone know the gain of these) as a solution. Do you have recommendation for other power amps that have a warm presentation that might suit? Does it matter if amps have different power ratings? Audiolab's are 100W, would you recommend more, less or same power for tweeter?