Do you use a separate amplifier for zone 2 or is it built into your avr?
If it is built in and your avr doesnt have low level outputs for zone 2, than you need a sub woofer with an internal crossover that accept high level inputs...
If you have low level zone 2 rca outputs, simply use an rca splitter and plug an rca into your outdoor subwoofer, again you will need a sub with an intergrated amplifier and cross over, which most have...
PS outdoor subs are awesome my brother has 2 buried subs, by his pool, they sound awesome...