Am I allowed to resit just one AS level subject?

As far as the examiners are concerned you are free to do whatever you like.

Your school can advise on whether they have any rules around progression to A2 study.

I just finished taking my AS level exams in November and have progressed to A2 level whilst waiting for my results that will come out in January next year (2024). I have doubts that I might not do well in my AS level Biology. If that turns out to be true, is it possible for me to just resit Biology AS level for that one subject whilst still progressing in A2? Or must I resit for all the other subjects that I took in AS? Is it also possible to resit AS while also taking A2 in the same exam season (May/June)?

You can resit an individual AS-Level at the same time as your other A-Levels or AS-Levels. I presume you're taking CAIE? 😧