i think you would have your coding agent work on both code bases at the same time. You could also task with generating identical tests for each platform. It's easier for a model to keep up with that kind of tedium than a human. Plus you can just tell the model to keep re-doing things until you're happy and it won't quit.

Also, having agents trace and document every logic path to compare with another codebase works well in my experience. It can certainly do that better than me, i would give up and start taking shortcuts pretty early in a process like that.