I was speaking with a friend about stacked Advents the other day. I was curious to know if he had tried them with the top speaker upside-down, so the tweeters would be together in the center and he said he had not. I thought it would have been interesting to do that and see how it compared to the bottom-to-top configuration that he used. Now you can try it and see. It may help to put them on short stands to get them off the ground a bit and to put a vibration-absorbing maretial between the cabinets for isolation.
I don't remember if the Advents were 4 or 8 ohm speakers, but if they were 8 ohms nominal, then most amps would do fine with a simple parallel hookup. If they were 4 ohms nominal (I believe the small Advents were, not the large) then you would need to consult the amp's manufacturer because a parallel hookup would result in less than 2 ohms impedance at some frequencies and not all amps can handle that. In that case you could always try a series hookup, but then you'd likely need more amp power.
Happy experimenting and let us know how you fare.
I don't remember if the Advents were 4 or 8 ohm speakers, but if they were 8 ohms nominal, then most amps would do fine with a simple parallel hookup. If they were 4 ohms nominal (I believe the small Advents were, not the large) then you would need to consult the amp's manufacturer because a parallel hookup would result in less than 2 ohms impedance at some frequencies and not all amps can handle that. In that case you could always try a series hookup, but then you'd likely need more amp power.
Happy experimenting and let us know how you fare.