Probably --maxdualimpact *,0,0
where * mean exclude, and 0,0 mean default value
--dualdevices didn't work and it will not as stated here:
Use --dualmode TONDUAL in order to activate this mode.
The parameter --dualdevices is meaningless for this mode.
Not 100 % sure but ...
or wait for Jgonzi...
EDIT: I circumvented the problem by just using another miner for the first card, either way thanks for the help.