wouldn't think so. both work on gaba receptors so if anything they might potentiate eachother. but someone here might know of some contraindications, just that I personally wouldn't think there would be any bad reactions besides the potentiation factor.
and p.s. if you take phenibut on a daily basis, I would keep some of that baclofen handy because it will help in the event you run out of phenibut and go through w/d's. phenibut has pretty nasty w/d's, some say it's on par with benzos.