can't see any problems with that at all, known many folks that used Heroin to some down from MDMA and never saw a bad reaction, and given they wrok in such different ways I can;t see it being a problem. But I'm no medical expert so I'm sure someone more knowledgeable will chime in before you get around to taking it.
I checked the drug interaction between Heroin and 5-HTP online and it said it was fine no MDXX was in the data base, but I think if your OK with 5-HTP I can;t see how MDMA is going to so you any harm especially with a break in between, lots of folks on here do them at the same time.