The potency of the shrooms and individual reaction vary. The first time I did shrooms I did a whole 8th and didn't smoke any pot, I got some nice visuals and fucked up thoughts but was in full control of myself.
The next time I did an 8th from a different source and smoked, became totally incoherent. What I do remember of it I could have sworn was just a dream.

If you're going to do it you should probably just start with half an 8th, wait 2-3 hours and if you don't feel you are tripping hard enough do the rest. If you feel you have had enough then you can use the rest for another trip later.