Honestly, I would just put both on there.. as the other dude said, it'd half the voltage output from the headunit if you only put one on.. and the chance of it being one channel per coil is pretty high... I'd just not risk it and put a Y splitter.. it's only like 5 dollars extra.. Not really that big a deal is it? just y split at the back of the headunit (pretty messy, but you don't have speaker amp yet, so it should be fine) and then normal rca out from there.
I'm more of a 'why not spend the extra dosh if it'll make it sound better, or reduce the chance of it sounding like ass' or something along those lines.
Better to have spent the extra money on the y splitters than to have not and been disappointed with it... Imo, i would do it anyway.