Here's an oddity I came across. An electrician was called to a house, because of lights flickering. He came to believe there was an open in the neutral of a 2-wire branch circuit. But he was unable to locate and correct the open. So, he jumpered the neutral to the neutral of another 2-wire branch circuit, in an existing outlet box.
I don't know the code, especially chapter and verse, but I think you'd have 4 bad results: a) depending on the phasing of the other circuit, if it happens to be the same phase you could get an overloaded neutral, b) Violation of that code rule saying "no paralleled wires (except for a few well restricted exceptions)", and c)the other circuit's breaker needs to be ganged with the first circuit's breaker. And you'd have current passing thru the hot wire of one cable (raceway) and returning thru a separate cable (raceway) neutral wire (because of an open on the first raceway's neutral) which I'm sure is a 4th violation and if not, still undesirable. You'd have a large loop of 60Hz magnetic activity happening, and in severe enough cases you could get magnetic inductive heating of connector and box hardware.
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.
The OP needs clarity, a branch circuit by definition has 2 conductors so stating it was a 2 wire branch circuit was a bit misleading I made the assumption he was speaking of a MWBC either way a handle tie breaker would be required a simple voltage check at the box would tell you if you were on the same phase.