07 Mustang Shaker500 head unit swap questions

[Lhnetman]

I’m replacing the factory Shaker500 head unit with a Kenwwod Excelon one. The factory setup has 8” sub in each door. The Metra harness that is needed has a rca that plugs into the aftermarket “sub out”. My question is how do I add a sub in the rear? Do I just “split” the output(y-adatpors) so both amp receives the signal?

Thanks