Not all MB is equal.
If the cab for the MB210ii is a passive one that connects from the speakon 8 ohm output on the amp, all good to use another amp.
Whatever you do don't hook up the combo to that set in reverse, or you will have two amps needing repairs.
If it's an MB Active extension with it's own amp inside connecting from the combo chain out XLR and plugging in it's own power cable, not happening. Maybe the active can be defeated and there is a speakon for connecting an amp. Doubtful.
Best post a photo of the extension cab back panel to be sure.