Hi NoNo
Guineas will eat the soft pooh of another pig (the first passing) sometimes if they are unwell . First pooh is a good probiotic and contains alot of the nourishment that isn't absorbed the first time through.
As a vunerable prey animal, they are designed to fill their faces when they can then go through their pooh eating at leisure when it's safe.
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You usually don't see the first pooh, they'll sit there, glare, lift up their body, duck their head then, gone. Ours give us a judgey look like we're going to steal it 
WRT your piggies, I'd be worried that she has a wound or irritation and the other one is grooming her fur to ease it. Maybe the pig can't reach herself.
Depends on the piggies how often you clean their cage, I read about people who clean the cage weekly and either they have very tidy hogs or they're up to their gills in pooh and pee.
We got two adult sows last year to match with our boar (neutered) and they pee far more than the males did.
When we had the 3 together, I gave them fresh newspaper and hay daily. They pick one patch to wee in and it does get saturated, I really couldn't leave them more than a day.
Sows can pee against the side ,we've had to wash our GP4 rump a few times, just a bum dip, to keep her comfy. (Though we suspect GP3 was pouncing, sows can pee to ward off unwanted advances 
. Poor boar )
So, ............ try daily cleans, maybe a bath while they're indoors. ?
And have a check through her fur for any red marks or wounds.