umpatt06

I have a boat that I wanted to hook 2 amps to. 1- 2 channel for sub and 1- 4 channel for 4 speakers. I only have one pre out on my deck so i used y cables to split. When I hook up 2 speakers everything is fine. If i hook the other 2 none work. I noticed if I just connected the positive and not ground then the other 2 speakers will work. (2 speakers normal, 2 speakers without ground) I am trying to figure out why this is so.

Please help.