As far as it is recommended to put the same model in the second slot for dual channel to work ... its actually only fail safe recommendation.
Preferable same brand with the same spec's... but as far as I've seen totally different ram sticks working fine after few tricks I'm just guessing putting totally other stick would have to be a test and trial.
In the worst scenario it would be recommended to be at least the same voltage, since other parameters can be usually adjusted in the BIOS, thus setting them up based on the slower stick would probably be enough.