Second Sump Question

[phar]

So, I'm thinking about adding another sump to my aquarium. Here's my thinking. I have a lot of live rock (~170 lbs) and a ninety gallon tank. I have a 40 gallon sump underneath the tank and it's already full of live rock. I want to free up more space in my display but don't want to get rid of the rock. I have an extra 27 gallon tank kicking around (not drilled).
I'm trying to figure out how I can use this as a second sump but move water back and forth main sump to this second sump without creating a possibility of a syphon. (My concern is a power outage will flood my basement...)
Any advice, or should I just sell the extra liverock.
Thanks,
Patrick