hmm this is strange, after i restarted the miner it has no cpu usage in the beginning, but an increased memory usage of 118 mb and 115 mb.
After like 3 minutes the 5850 Miner was again at 25% cpu usage.

i also noticed that it was the diablo miner for the 5850 who was using 25% cpu usage and the 7870 was at 0%.

But as i was starting both up, i noticed that at the 5850 there is an line "BFI_INT patching enable ...." and on the 7870 it isnt.

this is my starting the D1 is the 7870

start DiabloMiner-Windows.exe -o localhost -r 9332 -D 1 -f 120
start DiabloMiner-Windows.exe -o localhost -r 9332 -D 2 -f 1

EDIT: chaning the f 1 to f 10 did the trick!

thanks for your help guys!