Tim writes:

Any mismatch is destructive, and eventually, they will mismatch.

I really don't know what you mean by "destructive"

Two equally good batteries should work fine.

No. Currents are at best unstably balanced. Any mismatch (difference)
in the cells (size, manufacturer, model, flooded/sealed/gel/etc, date of
manufacture, age, prior application, usage history) at the start or
developing over time will result in loads between the batteries
themselves, which over time will accelerate the mismatching process.

A shorted cell, not uncommon, will result in enormous currents that can
cause a fire or explosion.

I know it is done, but only as a compromise, and a bad one at that.

I would at least put fusible links in series with each battery before
the parallel connection. That will at least give some protection
against the fire and explosion hazard.

Think of it as yoking a strong horse and a weak horse to a cart side by
side. The strong horse will be spent dragging the weak one. The weak
one will be spent just trying to keep up with the strong. The power to
the cart is a fraction of what the team is exerting.