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.

but you can turn on the amps driving the speakers not being used, and the inaudible hiss that is barely exiting the non-playing speakers will cause it to not respond nearly as much as it would otherwise, almost eliminating the problem. it's not an ideal solution, but it can help.

I also agree with Sean's point of reradiating, not a good idea to have a "live" pair and a "dead" pair stacked. If you can't move them, the next best thing would be to get a sound-deadening cover or wrap, for the pair not in use. This may not be perfect, but is alot better than the free interaction you get now, and it may be more convenient for you to not move the speakers around. Removing them from the room is a better solution though. While on this subject, when working in a high-end audio shop, we did a fun experiment. We allowed customers to audition a speaker, and then without telling him, we then brought a cordless telephone in and set it down on the table, facing up. We acted like we had just finished a phone call and were just putting the phone down. We then played the music again, and invariably they would comment that the system didn't sound right. We used this demonstration in a "blind" way, to show the customer the effects that even a small speaker in the room can have on the sound. TV's, small radios will all have some effect. Eliminate or cover all dead speakers in your listening area and you will have better sound.