It is possible, but you have to get the timing exactly right. Otherwise the DXM Hbr is just counteracting the slow release. I've done the combination 3x, but honestly never been a fan of combining the two. Too long of a mellow trip and for some reason taking that much of each product just makes me want to hurl. I honestly like my niche in the 3rd and 4th plateaus with my trusty DXM Hbr, but please don't just 'jump into' these plateaus if you are relatively inexperienced to DXM.
Are you planning on taking the polistirex (slow release) from Delsym? It's not too bad tasting, kind of like creamsicle. DXM polistirex is nice because oftentimes it is the sole main ingredient in syrups. This means you don't have to worry about APAP, guaifenesin, or doxylamine. But for precaution always triple-check your ingredients and dosage calculations.
However, for the sake of your stomach and your beautiful non-dyed intestines let me tell you how to extract polistirex DXM from delysm syrup. Take an empty gallon milk jug and fill it about 3/4 the way with water. Then pour your desired amount of delsym into the milk jug. Put the cap on and shake shake shake!! Let the milk jug sit undisturbed for at least 24 hours. You see, DXM poli is just DXM coated in a plastic 'bead'. After 24 hours you will see the DXM poly on the bottom of the milk jug. Carefully move the jug to a bathtub and start cutting slits from the top down to drain the extra fluid. Once you get to the bottom you can now take a shot of your desired DXM poli dose.
*please note this extraction does nothing in terms of DXM Hbr or removing APAP. This ONLY separates DXM poli from the junk syrup.
As for when to take the DXM Hbr in relevance to the DXM poli, there are several threads on BL explaining this procedure. A simple engine search should suffice for proper instructions. Good luck and safe Dexing!