If a creature that has swallow whole has a creature already swallowed, can it keep doing swallow damage as an action?
Well, technically no. The creature doesn't decide to continue doing the swallow damage or not.
It happens automatically. Along with a few other nasty things.
A swallowed creature is grabbed, is slowed 1, and has to hold its breath or start suffocating. The swallowed creature takes the listed amount of damage when first swallowed and at the end of each of its turns while it’s swallowed.
So it is similar to persistent damage. The swallow damage keeps happening at the end of the swallowed creature's turn.
Also, some PCs will have to switch weapons, which can be rough when Slow 1. If you drop a weapon, then it'll take Swallow Whole damage! So they should stow and draw, losing a round, and all while using up their air supply.
Also, some PCs will have to switch weapons, which can be rough when Slow 1. If you drop a weapon, then it'll take Swallow Whole damage! So they should stow and draw, losing a round, and all while using up their air supply.
I like wearing a spiked gauntlet to be prepared for just such an occasion.
If a creature that has swallow whole has a creature already swallowed, can it keep doing swallow damage as an action?
If you are asking whether a creature with the Swallow Whole ability can swallow more than one creature, the answer is that it depends.
If a swallowed creature is of the maximum size listed, the monster can’t use Swallow Whole again. If the creature is smaller than the maximum, the monster can usually swallow more creatures; the GM determines the maximum.