According to TURTLE :

Hey you say it is legal but the fellow ask if the breaker set up in a set. Will
one make the both of them trip properly as if they were seperated. Not being
smart at all but it even being legal will each breaker trip properly as one side
getting over loaded and the other side not over loaded ?

tie-barred breakers (whether manufactured that way, or later add-on)
will both trip if either of the breakers overloads.

That's the whole point of tying the handles together.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.