Note that Smode is bad to reconnize on wich GPU it will works if they have the same model,
Two different GPU from AMD should be the best.
Else Primary output do the Jobs.
Note that on extreme case with many DVI adapter and some DVI grid, we encouter some issue with WX7100 Display Port alimentation.
Everything was solver by using WX8200. This may be a driver Issue...
Hoping all of this can helps
Best regards.