Stacking subs to save space


I have a pair of Focal. Aria 936 and a pair of REL S3 sho's with a TV stand in the middle. This takes up a large about of real estate in my living room so I  had the idea of putting one sub on top of the other to save some space and wanted to know the downsides to it. The subs are wired right and left channel off my primaluna.

Thanks
Steve
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Some subs are designed to securely stack (eg, some higher end RELs) but for those at aren’t I’d worry potentially about shifting due to vibration, as one potential downside. 
Stacking them will permit them to couple and slightly increase output.  OTOH, they will both be subject to the same room-boundary effects, effectively increasing the magnitude of those peaks and nulls.

I'd prefer them separated and placed asymmetrically (and EQ-ed).
I would also think that the vibrations from the top sub would affect the bottom and or vise versa. Anyway to place them side by side with a separation between them?

ozzy
You can probably use some feet or material to make sure vibrations don't cause the top sub to shift off the bottom one, and I would assume the existing feet are sufficient to deal with most of the vibrations.

The downside is really the loss of smoothing out the bass response due to having both subs in one location, as kr4 mentioned.