Don't understand why you think they would not be in phase. Each identical amp fed by the same input would have negligible output phase shift difference between them.
Actually I am sure that there would be an out of phase at the final outputs by using independent amplifiers, though I do not know if it is negligible or not.
Anyway If I do not find another solution I will use the independent amplifier method.
Is the output of 2 independent amplifiers connected to 2 independent speakers side by side just 2x power of each? For instance what about using 2 LM3886 each delivering 68W?