According to Toller :

That's a good point. They make quad breakers for my box that nest two 240
circuits. Perhaps it would be possible to put a tie through all the
handles. Unless of course that stopped the breakers from tripping properly;
then it truely would be dangerous.

I would not expect a quad breaker of that type to operate correctly
with the two halves tiebarred.

Really the question an understanding that "safe" and "unsafe" aren't
absolutes, and the real question is "how unsafe are you willing
to tolerate"?

There are several unpleasant possibilities with such a circuit.
Is it beyond your toleration level?

I think it should be. But that's an opinion, not an absolute fact.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.