Hello there mate!
Get a 4 channel car amplifier and a 4 channel 4 gauge wiring kit; Make sure your amplifier has variable high pass/ low pass settings;
With a four channel amp you can run a set of RCA's from your head unit and then using Y adapters, you can add 4 channels to a two channel RCA outputs.
Alternatively, you can buy a high to low converter and use the front speaker outputs from your head unit to have a low (RCA) output;
By taking this approach, you would be able to use your fader from your head unit to phase out front and rear channels to your liking.
Most 4 channel amp will handle a two ohm load so that is no issue there but since your front speakers are 4 ohms and the rear, 2 ohms, the sound will be shifted to the rear of the car and since 6x9 speakers are more efficient than front speakers, you will have to set the gains on the amplifier so that the front sound stage is louder than the rear speakers. Once it is all installed, just play around with it until you set it to your liking.
Also use the high pass filters for your front speakers, say at 80hz and then experiment with the rear until you are happy with how it all ties up, You may run the rear at full range but that depends how you would like your sound. Engaging the low pass on your amplifier to run the rear 6x9 JBL's will run like 8 inch subwoofers then you will get vocals from front and the rear will provide the 'thump'. Whichever way you decide to go is you choice as you should enjoy your sound system. I did not speculate on a particular brand of gear since I left that to you to determine once you go to a car audio shop and advise them of using your speakers to run them.
If you are not competent on running those power/ ground/ remote/ 2 x sets of RCAS in your car then best to leave it to the experts to do the job for you.
You will also get warranty for the install so if you do have any issues you can always contact them.
Good luck with it all mate!