| I've got both of my amps grounded to one of the two bolts that holds down the trunk latch. in most installs i've heard it's best to have the same grounding point, less chance of introducing noise into your system is the reason why i believe, if you aren't getting any problems with ground loop/noise related issues then i see no reason to have the same grounding point. In my own experience, just make sure your ground wires are as short as possible and make sure your ground is as secure as possible, i had a loose ground once and while installing sound deadoning material i accidentally removed my ground wire/or loosened it it not sure exactly which one and i fried a hide-away dsp that i had for my HU.
As long as your ground is secure and you aren't having noise problems you should be fine with two grounds. Anybody feel free to correct me if i'm wrong since i am no expert. | 
