Ohh dear my robot has a problem

[Laser]

Ok I have 2 different voltages in my robot 18v and 12v.
I want to only use 1 link for my robot so I am using negative as ground.
I put the 2 battery's GND together than have a link and the the wires leading to the sabertooth and the electronize.
When the link isn't in the sabertooth lights up, but shows error.
The motor running on the electronize spins very slowly.
I think something has gone (majorly) wrong.
What should I do?

[kane]

If you have your link on the ground a circuit could be created between +12v and +18v. I would recommend running the robot from 18v and just trimming the speed controllers to reduce the output power. This makes the whole setup simpler and less likely to go wrong. If you are running dual voltages it is safer to isolate both circuits seperately.

[Laser]

Ok I'll run it on 2 different voltages and have 2 links