Yeah, but delsym absorbs kinda slow. It's hard to hit a third plateau with it.
The "secondary" dose will not be metabolized as readily into DXO because of the DXM already in your system. So he'll have a greater ratio of DXM to DXO. So this will be less trippy and more delusional... He'll still be intoxicated, but it won't feel like a well-defined euphoric 2nd or 3rd plateau. DXM-heavy intoxication makes me feel sketched out.
The more DXM you have in you, the longer it'll take to metabolize off and thus wear off. If I'm indulging in 1.5 grams, it's a good 24 hours 'til I come down fully. I'm tripping hard for 10 hours, and high for like 14. Then I feel burnt out and hollowed out for like 2 days after.
I'm a fatty, so maybe my metabolism is just slower than normal though. You should post your results SEMO.