First of all, sorry for my bad english, I will try my best.
I have two API's using the same database, in theory for distinct purposes, one for the user (edit profile, get user info...) and another for finance (store a payment, store an order...). I have a problem with this architecture, because I don't know what I do when I make a function that I need in both API's, but only one have it, if I perform a request and set a cors protection or if I duplicate the code... Any suggestions are welcomed, but keep in mind that I cannot change the architecture.
I would make a library out of whatever function you need in both, then deploy it twice. That way when they diverge, it's an easy change, but you minimize the cost of doing it twice.