> If the position is that one of these models will go out of control and destroy humanity then why would having two such models be safer than one?

Models don't go out of control, they generate totally harmless tokens. The people directing the models go out of control. The owners of the single powerful model are seriously tempted to go rouge and they're going to do it, sooner or later.

Two or more competing groups force the development of defensive capabilities which require strong models too. In addition, offensive capabilities in possession of different competing groups introduce the option of MAD (Mutually Assured Destruction) which is a form of defense that doesn't require defensive capabilities. Worked with nukes.