In my experience breeding cockatiels and budgies, most birds will lay 2-3 eggs before they sit on them, while some birds will start sitting on their first egg right after laying it, though that is not usually the norm. My budgie breeders over the years, both English and American, rarely ever sat on their eggs until they laid at least 2 or 3, while my cockatiels were usually the same, though the couple of birds that I remember sitting on their first laid egg were cockatiels. That being said, during their very first clutch, the birds that sat on their first egg right after laying it ALWAYS sat on their first egg laid in EVERY clutch. So as already mentioned it definitely goes by individual birds, and however they do it seems to be how they'll do it every time they lay a clutch.
Also, if it's the very first clutch laid for a pair they may be a little "scatterbrained" about what they are doing, lol. Just like the very first clutch for a pair may have most or all of the eggs infertile because they haven't quite gotten the hang of "things" yet, they also may not get it together until they have laid two or three eggs, and then they realize they need to do their thing.
Just watch both clutches, it's nice that you have two separate pairs laying clutches simultaneously so you can compare what is going on and their timing. I'd say if that hen lays another egg and doesn't start sitting on them at that point (by the end of the day she lays the second egg, make sure you give her some time to eat, drink, poop, etc. after laying it before you take her eggs away) you may want to consider fostering them to the other pair that is sitting on her clutch.
"Dance like nobody's watching..."