I know the Equinox AV is designed to be used as a maximum three-decker – ie a base unit and an extra shelf module on top – , but I suspect that's because of stablity issues should you pile it really high and then put a socking great heavy 50in plasma on top.
I'd suspect with a relatively light load, such as just your centre speaker, on the top, and keeping anything heavy – such as the AV receiver – on the bottom shelf, you might be able to get away with a second shelf unit on top of the Equinox.
A word with Atacama - it's 01455 283 251
or email
post@atacama-audio.co.uk
– explaining what you want to do will soon find out whether I am barking up completely the wrong tree here, at least in official terms. But we've used taller Equinox stacks in our listening rooms with no problems.
Consulting Editor, What Hi-Fi? Sound and Vision / whathifi.com
Audio Editor, Gramophone