You wouldnt have 2 subs in most cases. Just one. Most subs have left and right balanced ins and outs.
You would go monitor out l/r from your mixer to monitor in l/r of the sub. Then monitor out l/r from the sub to your monitor left and monitor right inputs.
I have the yamaha sw10 sub and h80m monitors connected up this way (and an excellent choice for budget monitoring).

Sent from my GT-I9100