i would not recommend that to connect your sub on the same port of your sound speakers.

i have 2 amps, one for my 2 subs and 1 for my 4 speakers. This because i turned off the bass of my speakers, this because i have enough bass of my subs.

 

So what I did was I took out my door speakers and put my new speakers on the old wire which was going to the head unit and ran my rear speakers to the amp and my sub to the head unit because it's a active sub it all seams to work but how long I also don't know if I connected my rear speakers the right way I used to bridging channels for the two rears