You guys ^ posted at exactly the time - what a coincidink, eh?
Anyway......after plugging in two subs, now you get to figure out how to run them in proper phase together - better down a couple beers first for this fun activity! And the sub distance setting on your receiver (really a phase control in disguise) will be almost useless now unless both subs are located the same distance from the listening position.
A "Y"splitter is the easiest. But you can get more in depth. I have my LFE through a "Y" splitter and then to a BFD equalizer. One sub is center front and the other is center rear. The right channel is used for the rear sub and the left channel for the front.
If the two subs are located together, use the splitter at the sub end. That way you don’t have to run two long cables from the receiver. You can run one long one, and jump from the first sub to the second with a short cable. Electrically, it doesn’t matter where you split the signal, so no reason to spring for extra cabling.
Also on the auto calibrating receivers or pre/pros... I have found if you do one at a time and if there is a difference split the results down the middle then tweek each sub to taste... Its really not that difficult unless you have a really weird room.