It's been a while since I've busted out this knowledge so I'm a bit rusty, forgive me fellow blue-lighters if I mislead; I'm only trying to help. Theoretically, they have somewhat contradictory effects, that is to say that ritalin might inhibit adderall's action a bit through its reuptake inhibition. Practically speaking, from what others have mentioned, they don't seem to cancel each other out to the degree one might assume, and the combined effects will be noticed.
It's worse for your heart, too. That's for sure.
That said, at the very least the good news is that ritalin has been shown (on BDD I am not quick to find & cite, sorry) to ameliorate to varying degrees dopaminergic neurotoxicity induced by adderall.
I imagine it would FEEL very much like a roller-coaster ride, heh. Ritalin's halflife is very short and I happen to find it does its thing and dissipates quite quickly, as opposed to adderall's elongated, more consistent action.
That said, I've never actually combined the two, though I have taken both separately. If you're looking for real-world experience from users of this combo, I can't help you, and can only offer the bump.
EDIT: the search function on this forum is very useful and I was certain I could find some older threads on the subject. Further reading:
http://www.bluelight.ru/vb/archive/index.php/t-283068.html
At a glance I believe this corroborates much of what I said.