Turn the music signal from one amp 180% out of phase compared to the other.
Then connect the two +'s of the speaker outputs of the two amps to the + and - inputs of the single subwoofer.
You don't use the two - speaker outputs of the amps.
So your subwoofer sees; double the output of a single amp, but beware of the dangers, this is getting serious.
If you're not completely confident of what you're doing, contact a local caraudio dealer (with experience with SPL cars).