The amp (Kicker IX405D) has a sub section which outputs 390W (Dynamic Power at 14.4v / 290W (RMS at 12.2v)

As yet, you've not said what load impedance these power figures are delivered to as specified by the manufacturer.

If it's specced for 2 Ohm loads, then the power developed in your two subs is correct. If it's into 4 Ohms, then a 2 Ohm load will only result in half the designed power being delivered.

Hence, I would guess these power ratings are for 4 Ohm loads, and you would be better putting the two subs in series to give a total 4 Ohm load.