I use a ati 7950 GPU and tried to put an older 5850 gpu to do some Boinc work.

It is different drivers for those two GPUs. When installing the legacy driver for the 5850, Boinc would only use the 5850 . I made the cc_config.xml file for boinc to use both gpus. AfterrReinstalling the latest driver (for the 7950) the 5850 is gone.

Hardware monitor finds both but read only data from the 7950, while it found and read data from both cards after I installed the driver for 5850.

Is this an impossible task? Maybe it is not worth it anyway but I guessed that the 5850 would crunch something like 5000+ credits per day extra.

Maybe it is not worth the effort. The pc is silent even 100% load 24/7 and total quitet if I pause Boinc. With the old 5850 added it turned noisy even when only one GPU was ever in use. I put it out from the cabinet and gave up the whole thing.