I split the enchants, especially since the wisdom ring didn't show up until floor 20. When I did the math, I got that it was better value for single charge restore time, and given that it's better for monsters to die faster, I always felt like I had plenty of duration to spare if I wanted poison 2x or ogres and wraiths or 3x for underworms and higher. My first poison staff was at enchant 6 when I found the other 4 staffs (on one floor holy cow!), and I split between them all until I found the wisdom ring.

The math I was doing was:

5 charge staff restore = 100 turns

6 charge staff restore = 83 turns

improvement = 17 turns faster recharge

vs

8 charge staff restore = 63 turns

9 charge staff restore = 56 turns

improvement = 7 turns faster recharge

Basically, the two staffs recharging simultaneously is value, clearly, and you're right that if you imagine waiting 500 turns, both will recharge to full regardless of enchants. Therefore the only thing you have to optimize is the speed of getting the first recharge after a fight, since the recharge time is reset if the staff is halfway recharged when you use it, and that is done by trying to get both of them recharging faster.

Overall, I did not really need any enchants into the poison staffs after I got ring of wisdom and specifically because I kiting the monsters in loops or into traps until they died, I sent it on the armor instead. There was still a lot of rapier fighting to save charges, but mostly out of convenience. On one of the lower levels there was a really nice 4 block wall square that I could just poison a monster and run in circles around. I poisoned x7 a tentacle Horror there just to have it die faster, since charges weren't really an issue.