As Rick says, it's not quite a simple as wiring things together. There are differences in how the input signal needs to be presented to each amp.
Here's how to wire it; you see that input A gets the 0 phase signal, input B sees that same signal 180 degrees out of phase (inverse signal) That phase reversal is done by the transformer. The problem is that you likely will not have the splitter transformer built into the input stage of the head unit amplifier.
Most external power amps for car audio will have a bridge mode, but it's pretty rare to see it in the head units.