It is easy to put two images side by side using the Transform tools in FCP. But if I understand correctly, you then want to scale them up. The problem is not to do it simultaneously, that is easy. The problem is that - assuming I interpreted correctly - you need the each image to not overflow its frame.
You could do that using yhe Crop tool, but if I were to do it, I’d rather create a custom Generator in Motion. I may have a go at making one, if you like, but probably not until late tomorrow evening or may Friday. Let me know if you want me to have a go at it. If I do it, you can use it freely but of course no guarantees.