problem with grounds

I want to conectthe basic stamp to my h-bridge and there is a problem because when i conect only one wire to the basic stamp everything is ok,but when i conect both wires it won't work.
There are two wires to which has to be connected power in order to turn motor in a direction, it's a ground, but they can't be connected at the same time , i mean that one wire is to go foward and the another one is to go in reverse, so they can't be connected at once. and ther is a problem because i'm using tip31 as a switch from basic stamp to switch the power into one of the wires, but it doesn't work,I was thinking about diode betwen the grounds of the transistors, would it work?, I'm not sure if i good explained it , but i hope you get what i mean,
thanks

Have you tried an H-Bridge? that should do what you want.