In parallel you divide current, theoretically, not voltage. But what happens is that the two roughly 1000 ohm input impedances of the two subs, when connected in parallel, look like one 500 ohm input impedance to the preamp. At 2 volts, it's the difference between .002 and .004 amps.
Either way, though, a person will use the sub level setting in the receiver to cause the receiver to output the appropriate voltage to obtain a balance between subs and everything else. A typical fix for auto-on subs that won't wake up soon enough is to use a Y cable to connect both left and right inputs (if available) so a little more signal gets to the sub . . .