210.4(B) is for ungrounded conductors not grounded conductors.
The OP said he was dealing with 2 MWBC so these 4 wires now comprise the MWBC so all 4 breakers would need to disconnect at the same time its a subtle violation.
I think you should go back to the OP again! Two (2) wire branch circuits IMHO are a hot & a neutral. Therefore he now has two hots (of unknown phase) and one neutral.
Yes, a 2 pole CB (single handle) would be required, and yes, the neutral could be overloaded depending on the phasing, but that is unknown as of now.
It's a dangerous practice to do what the OP found.