since one is a stim and the other is an opiate, you're basically speedballing
Speedballing is intravenously injecting a strong stimulant and a strong opiate in the same injection. MDMA is an empathogen, not really a straight stim. It's not the same, and it doesn't carry the same risks.
My understanding is that speedballs kill because, when the cocaine wears off, the full effects of the heroin come into play. Fentanyl is a very short acting opiate, and its action would be over long before the MDMA wore off (that is, if it is smoked or snorted, the patches are time release).
Fentanyl is not the strongest synthetic opiate (opioid is what they're called). There are analogues of fentanyl, such as sufentanil and carfentanil, that are much more potent, as well as unrelated drugs. Fentanyl is not synthetic heroin; heroin is diacetylmorphine, whether it's produced from poppies or synthesised in a lab. Fentanyl is a different drug.
Whether the patches can be cut or not depends what kind of patches the OP has (if he has patches at all, he said he had a bottle). The matrix patches can be cut up for buccal or transdermal use; the reservoir patches cannot. However, the gel in the reservoir can be smoked, or diluted with water for use as a nasal spray.
you die of a very painful overdose
It wouldn't be a painful overdose...after all, fentanyl is prescribed for cancer pain.
I personally adore the combination of MDMA and opiates. There's no speediness, or any kind of stimulation really, just warmth, and contentedness, and euphoria. A powerful sensation of well-being. It's very chilled, but wonderful. I feel like a king on this combo. Just don't dose too high on the opiates.