what should I do with my 2 chanel amp?

My vote would be to amplify your door speakers and leave your rear speakers off the head unit or disconnect the rear speakers all together.

My reasoning lies within the answer to the following question: Do you turn your back to the stage when you attend a concert or live performance? If the answer to this question is no, then you probably wouldn't like your speakers blaring from behind your head. If your answer is yes, then you are one of the exceptions to the rule

Is it hard to connect door speakers to amp? =/

You can actually use your existing factory wire if you want to make your life a little easier. Other than that, it is no more difficult than power, ground, remote, RCAs, etc. for your subwoofer amp.

I actually know someone who put a 150 x2 RMS amp on his factory speakers in a Chevy Silverado Pickup Truck. He has the gain turned all the way down and surprisingly, the factory speakers actually sound OK with clean power. I won't say they sound great, but I will say they sound better than expected.