Combinations - Combining opiates

[Ontherooftops]

A friend of a friend would like to know other's experiences combining mu-opiod agonists.

He is quite well versed in opiates, but tends to maintain a fairly low tolerance.

Normally, 10mg hydrocodone is a threshold dose for him. But one day he took 10mg hydrocodone with some tolerance, and was not feeling much if any effect. Somewhere about when the effects of the hydrocodone should have been peaking (though he couldn't percieve them) an accomplice offered 2mg hydromorphone insufflated, and the effects were drastically increased, resulting in one of the best opiate highs he's had, pretty much ever. Though there was less respiratory depression/physical effect, the euphoria was equal to around 30mg oxycodone insufflated.

Since then he has taken up to 12mg hydromorphone, but has never been able to achieve the same satisfying glow of hydromorphone+hydrocodone.

Could this be due to diffrent binding affinity to diffrent receptors, leading to a more comprehensive activation of the entire spectrum of opiate receptors? It doesn't make sense for two opiates to have synergistic effect with one another, does it?