You definately have to ground the capacitor AND the amps, that's for sure. If possible, ground them to different spots, not the same one. Connect the power wire from the battery to the capacitor. Then, the power wire coming from the capacitor, should be going to the input of the distibution block. On the output of the block, connect your 8 gauge power wire and the power wire for the other amp. Then just hookup the power and grounds to each amp. You'll also need to charge the capacitor before use, or you could mess it up. Here's what you do...
1)Remove the fuse at the battery.
2)Connect the ground from the chassis to the cap then to the amp.
3)Connect the the power wire (from fuse by the battery) to the cap then to the amp.
4)To charge the cap, get a 12v light bulb with leads. Temporarily connect the power wire from the battery to one side of the bulb (at the fuse) and connect the other side of the bulb to the wire going to the cap. The bulb will light and over a period of time (10-20 seconds) it will get dimmer. When the bulb goes out, the cap is charged. Basically, your going to put the light bulb where the fuse was.
5)Now you can re-install the fuse.
That should get you going!
I work at Best Buy in the Car-Fi Department... and installed my own system. 2- 12" Alpine Type-S subs, Power Acoustic 2-Farad Capacitor, and Alpine MRP-M850 amp.