I definitely agree that a disproportionately larger dose of a stimulant will over power the effects of an opiate (I remember trying to knock myself out when I was tweaking off MDPV by shooting massive amounts of Oxymorphone to no avail), especially if there hasn't been that many hours between taking each drug. But IME the amphetamine family seems to work better with opiates than drugs like MDPV or Methylphenidate, the only reason Cocaine is commonly combined with smack is because its so incredibly short acting. Both Methamphetamine and Dextroamphetamine also are capable of masking opiate withdrawals incredibly well for me, far better (at least with Methamphetamine) than benzodiazepines do-though I should also mention, when you start to crash from speed while simultaneously withdrawing from opiates, you're in for one hell of a nasty ride.
I was talking actually about this to Tricomb the other night, and he told me that on certain occasions stimulants were actually administered alongside Opiates in clinical settings in order to increase the efficacy of analgesia-I don't have the reference, but if Tricomb comes on I bet he could find it.