Enforce unique values across two tables

Question

Is it possible to enforce uniqueness across two tables in MySQL?

I have two tables, both describing users. The users in these tables were for two different systems p

Answer 1

Maybe not direct answer to your question, but:

You should consider rewriting your code and restructuring your database to unite those two tables into one.

The design you are trying to enforce now will complicate your code and database schema and it will make any further upgrade to other database software or frameworks harder.