Utilize APU + GPU at once

[Rolf]

I can use both, they work perfectly, but I can't use both at once.

Say what?

Have you tried running, say, oclhc-lite?
What does it say?

As far as I know, some notebooks disable the integrated gpu (in your case, it's the 7600G) when the discrete one is used.
You may have to dig into the power options.
Apart from that, I can't really give any useful advice.