How long is OK to use little potty?

Our 3.5 year old twins are approaching 1 year of being potty trained (Oh Crap method). For the most part, we're super happy with how things have gone and there are no issues around the potty time apart from one twin (A) who heavily prefers the little Baby Bjorn potty. Twin B is making good progress -- she has started to use the big potty a lot more over the last month or two. We have one of those step/toilet seat attachment things for the big potty and she uses it by herself no problem. She'll still choose the little potty sometimes but it's about 50/50 now.

However, twin A has only voluntarily used the big potty maybe 5 times. She says she is scared of the big potty but cant articulate what specifically about it scares her. We really don't have any issues with her using the little potty. Cleaning is a chore but not a huge deal. The main concern we have is that we are trying to get them into pre-K next fall and we'd like her to be able to use a big potty by then. Currently they are in preschool but it's only 3-4 hours a day and I suspect she just holds it through school.

Does anyone have any suggestions if we should push this more or just let it go and hope she'll keep trying the big potty over time? I was also thinking that as she grows it will just get easier. Thanks!