When a new psychiatrist looked at my situation he insisted methylphenidate ER was better then dex-amp for ad(h)d. I stood my ground in fever of dextro-amphetamin so we ended in the middle on both. They both worked, the phenidate didn't cancel out the amphetamine, at 2x20mg mph ER & 2x10mg dex-amp. But I still couldn't stand the peripheral CNS effects of methylphenidate.
Else I would say the best of both worlds, if you can stand both and the dosages are reasonable I guess. There seems to be no bad interaction there is some medical background on this combo. Ehm, are we talking prescribed or recreational? At higher dosages I do see some concern, besides canceling out each other potentiation of some effects could be desastrous. Like the effects on the heart.