There's poly filter (aka filter wool and filter floss) and there's Poly-Filter with capital letters. I don't know why they gave it such a confusing name. The one that removes chemicals is made by Arcadia, though you'll sometimes see it sold as by Underworld, the company that made it before Arcadia took it over. It is also very expensive, which is why I just use one bit after all the carbon.
This stuff. I usually buy the 8 x 4 inch sheet which costs anything from £10 upwards. Poly-Filter (with capitals) is a sheet almost an inch think and stiff, with a hard feel to the surface. It is cut up with scissors to make it fit.
I would wait for 3 days after the last dose unless that would make it way past time for a water change. The medication will still be in the water for that time. Then add the carbon.
There is the possibility that your particular betta is sensitive to something in the second dechlorinator, so going back to the first one is a good idea. If you want to try one with the least possible chemicals in it, look for
API Tap Water Conditioner which contains only sodium thiosulfate to remove chlorine and tetra sodium EDTA to bind metals. Nothing else. No ammonia detoxifier, no slime scale promoter. However, very few shops sell it, I usually have to get mine on-line.
And the stuff that looks like cotton wool can be out in the filter, usually as the last medium, to remove fine bits that pass through the rest.