4 channel vs 2 channel

Hi everyone. I'm new and i need some advice please. I have a pioneer avic 7100 nex installed in my 2012 altima. The head unit is 50w × 4. I have Two 6.5" speakers in the front door and two 6"×9" speakers in the back deck. And I want to put two more 6.5" speakers in the rear doors. The speakers...

Okay i just wanna know how wrong i am here... If I'm looking to get 2 12 inch subs that run at 300 RMS dual voice 2 ohms, i want to get a 2 channel amp that runs at 300 RMS at 2 Ohms.. so then each channel gets 300RMS and/or a 4 channel amp at 2 Ohms that runs 150 RMS each channel then bridge...