I don't really find that aMT lasts as long as many people report. I found that with a 40mg oral dose, pretty much all effects had ceased within 9 hours and I could sleep after 12. Though this seems to be subjective as one person I did it with couldn't sleep for about 20 hours and another was nodding off about 8 hours into it! This was all without any downers, though I would recommend having some on hand. I didn't get a comedown but just felt physically drained.
I have done opiates about 6/7 hours into AMT and experienced no problems. I don't see how it would cause problems really. Though I'd say you should research it really. I thought doing opiates at the end of AMT was nicer than benzos as the latter tend to halt the 'trippy' effects quickly whereas opiates seemed to synergise better and the trip just entered a slow, dreamy phase rather than being quashed in its tracks! As with everything though, YMMV, as I know there are lots of people that like doing benzos at the end of a trip. I mean I'd take them anytime if there weren't any opiates around haha.