Any ideas for a grounding block to connect multiple TT to one amplifier? Is it a bad idea to connect more than one ground?

No problems with multiple groundings..

All grounds should be at a particular point. Everything should "Y" off of that point. The potential for a ground loop exists with any other set-up. The grounding terminals used inside of a fuse box should work, but will look ugly. Connect the grounds back to the preamp/receiver.