IMHO, I wouldn't run carbon and purigen at the same time. I probably wouldn't run carbon at all unless I had just dosed a medicine or something like that, but that's another issue.
Really really don't undestimate how small those beads of purigen are. The holes on the bag are so small, that if you just hold the bag say 6" under the faucet, most of the water goes around the bag. You have to hold the bag right up the faucet to see the purigen bits swirling around.
If there's no room in the first filter with or without carbon, then set up the second one. And purigen eats tannins for breakfast, so if you're not seeing your water clear up, it's probably because the water is mostly flowing around the bag of purigen instead of through it.
just my opinion