What happens when you try to grapple a creature that is already grappled by the sticky fibers of a web spell? Is there a chance that you will become grappled by the web? The spell states that "Attacking a creature in a web doesn't cause you to become grappled", but does that include grappling a creature in a web?
Suppose you successfully grapple the webbed creature - does that creature then suffer double the grappling penalties? If the creature wants to break free, would it have to break your grapple and the web's grapple separately, meaning that it couldn't accomplish this in a single round?
The creature would not suffer grapple penalties twice. Being grappled by two creatuers at once doesn't cause such a thing, neither should being grappled by a web and a creature.
I think this is a special case, and the rules don't adequately cover it.
How I would run it is, you would either have to pull the creature free from the web, or risk being caught (and grappled) by the web too because of how close you would need to get and how you would maneuver around them.
If the creature wants to break free, I would run it the same way as breaking free of multiple grapplers.
Multiple Creatures: Multiple creatures can attempt to grapple one target. The creature that first initiates the grapple is the only one that makes a check, with a +2 bonus for each creature that assists in the grapple (using the Aid Another action). Multiple creatures can also assist another creature in breaking free from a grapple, with each creature that assists (using the Aid Another action) granting a +2 bonus on the grappled creature's combat maneuver check.
Except in this case I would say that the the creatuer being grappled by both you and the web would need to beat the CMD that was higher between the two with a +2 bonus.
I looked at the rules for multiple creatures grappling a single target, but I don't think they apply. It's not two creatures, and they're not working together, so that "aid another" approach doesn't seem appropriate.
Of course it doesn't apply directly, but theres nothing else to give direction. The multiples creatures in a grapple is the closest rule you will get.
The important thing is, I don't think it will take two checks for the individual to get out. If one check is sufficient to get out of both, he gets out.