Methylphenidate + an opiate is probably considered a speedball, whose main feature is to mix two amazing highs (the stimulant's high and the opiate's high) in one. First you get the "buzz" of the stimulant and then the "peace" of the opiate, which eases the stimulant's comedown (this may be subjective but that's how it works for me).
However, a classic speedball is a (very dangerous) mix of cocaine and heroin (actually the original speedball used morphine). While cocaine and methylphenidate (the active ingredient of Concerta/Ritalin) are similar, cocaine is much more euphoric and that's usually the point of the speedball. So with methylphenidate's lower euphoria, the speedball won't feel like a traditional one.
Since methylphenidate's high is more of a "focus" high (depends on the dosage and ROA, tho) it might overlap the opiate's high.
TL;DR: When you mix a stimulant and an opiate, don't expect to feel both highs separately; they will merge into one different high, that depends on what substances you've used.
Also speedballing is quite dangerous, especially when cocaine is used, because stimulants up your heart rate while downers decrease it, so speedballing messes with your heart and can lead to heart problems.
