How to have one Auto Incremented id for two separate tables in mysql?
If the two tables share ID space, surely then they should have a common parent table?

I would create a parent table and make the two other tables children of it with foreign keys. The parent has the autoincrement field, and you have to insert there first before inserting the child. Add a "type" column to the parent table so you know in which table to look for the child with the given ID.

The biggest advantage to this approach is that you don't need any triggers. No performance or synchronicity problems to worry about.

In PostgreSQL you would implement this using inheritance, but I don't think MySQL has this feature.