Um, with one 10W7 in my car, it will hit 138db. You cannot sit in the car at that volume. Another idea - with the door open you can feel the rush of air out of the car as the bass hits, like a wind out of the car.
You don't need two, it would be overkill. The dealer will want to sell you another as they will be getting good margin on it, as opposed to just labour.
The thing to remember about the amp, it's not raw powwer, it is the way it delivers it and the control it has over the speaker. Smacking too much power to it will result in woolly bass.
Don't get downheartened by your effort. TBH 6 hours is nothing in the scheme of doing an install. The number of man hours spent on mine was well into 3 figures.....