stacking speakers?

Have you not tried it yet? You're probably in the best position in determining the implications.

The only real possible problem i can see would be a compromised tonal balance due to the height of the placement and placement relative to the walls, but you may get lucky. With HT, I doubt coupling differences between stands and your speakers will be a sonic issue, but watch out for scratches :)

Placing speakers that are being used near other speakers that aren't being used will cause "sympathetic resonances". In other words, the driver producing sound will excite the other drivers not being used. In effect, they would become indirectly coupled passive radiators. This could play games with quite a few various aspects of what you hear and none of them are good. That is why a GOOD "audio salon" only sets up the actual speakers that you are demo'ing and removes all others from the room.

Using that criteria, how many "good" audio salon's do you know of ??? Sean
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I overlooked Sean's good point. I assumed the monitors would be removed when not in use. Resonance is more likely to be a greater problem for music than HT in your situation.