You CAN'T use two SUBS, as this will overload it big time.
The purpose is that you can connect your left right FULL RANGE speakers as normal + and -, left /right.
The IF you add a SUB you use the BRIDGE connections ie - for the sub will be from one side, the + will be from the other side.
You do NOT have to use a pair of normal speakers you could JUST power the sub on it's own.
SUBS are very low impedance and it will blow the amp if you put one on each side OR two bridges.
Just one bridge, OR two normal speakers, or both together but NO MORE.
tabetha