I allow martials to "stack" for extra attack, up to the second attack. Any more than that requires actual fighter levels.

Paladin2/ranger3 gets extra attack, same as paladin 5. Paladin5/ranger5 still just has the normal 2, not 3. That's special for Fighter11.

This only works for martials, and no shenanigans with warlock, bard, or warlock will ever get stack this way. They (the casters) only get extra from their respective subclasses or invocations.

It hasn't broken anything yet. And in low level games, players can still multi-class and no "lose out" on of the few things martials actually do get.