Have you checked whether running two instances simultaneously is even possible in GrapheneOS? As @tomasz86 has doubted that here:
What you’re writing here is that the profile1 should show its own files? Probably a typo, so you want to duplicate all images between both profiles?
Overall I think Syncthing might be a poor fit for this use case. What you really need is a common folder shared between the two profiles, but all local to one device. Syncthing is made to synchronize between an arbitrary number of networked devices. Duplicating the data is just one problem with your approach, but Syncthing will waste a lot more resources in addition compared to a simple local folder which both profiles can access.