Both medications affect receptor sites. Asside from the addiction aspect, the problem is both also cause degradation of respirations. There is no easy formula to determine the quantity of the medications that will cause respiratory arrest.
I'm not sure, but I would highly encourage you to stay away from the methadone. I am a methadone addict and used subs to get off of it. I had to wait a long time, and get really ill before I could induce the subs. From my understanding, the partial-agonist bupenorphine wipes the receptors clean off all opiates and would only causes precipitated withdrawals if you where going into withdrawal from a full agonist opiate. As you are going into withdrawal from bupenorphine, not methadone, it seems you would be safe. This is a guess and I would ask your doctor if this is correct. Again, I give a friendly warning about mixing your drugs, especially methadone. You are playing with fire. If you ran out of subs, I would suggest some additional aftercare to help you stay on track and avoid this kind of mistake. Good luck and be careful.