I have read mixed opinions on taking cocaine and ecstasy together, a lot of people sem to think that coke dulls the MDMA high, but not everybody. Obviously methylphenidate is very similar to cocaine and one could expect the effects could be similar in this regard as well. It should be noted that this combination will put a good amount of stress on your heart, and there is a good chance it could be quite neurotoxic, meth and MDMA used in combination is known to be pretty damn neurotoxic.
I think it would definately be a stretch to call this combination safe, but if you are of reasonable health and have past experience using other stimulants with MDMA then this combination probably wont be a great deal more stressful on your heart. I am not a doctor though and you should really think twice before you combine these two drugs, most people would say that the MDMA high is wonderful enough on its own that is does not require the addition of methylphenidate or any other stimulant to be enjoyed.