Am trying to find a combined neutral in two 3-wire multiwire circuits. HO had a three melted bare return wires inside of boxes. resulting in a couple of white to box bonds. I am trying protect the wiring with 2-pole afci murray breakers. The circuits trip when one light is turned on it blows both afci breakers, but fine when the one light is not used. My thought is could the light blow both 2-pole afcis or just the shortest wire, this ones got me stumped....maybe lack of good sleep, et.al.
You need to rebuild the circuit -- logically.
With the damage reported, you need to inspect every point involved, because collateral damage could well be enough to trip out any make of AFCI.
(It's what they're designed to trip for.)
I'd meg the relevant conductors, all of them. Cooking neutrals/ returns to a box (assumed metallic) -- twice -- is a severe failure.
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I would strongly suspect that the DIY crowd has been in action, and that you're being 'screened' from the full history of these circuits.
It'd be easy for a DIYer to overload a give neutral by stacking phases on the neutrals -- in this case, twice.
BTW, if the panel is ancient enough, the modern phase alternation can't be counted upon back at the panel.
So, a DIYer following a youtube clip could -- and would -- shift the hots to the wrong phases -- IF the gear is that non-standard.
I mention this in passing, because such old bussing would not support any AFCI now being manufactured.
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As for identifying conductors, start with the hots. You have at least an opportunity to tone them in an ungrounded state. These should greatly narrow down the suspect neutrals back in the panel. Unless you're dealing with 'home-run' boxes, you should be able to break open a trivial sub-set of all the neutrals.
I'm assuming that this is NOT a Romex job. (Metallic boxes)
In which case, I'm assuming that you've got MC/ AC, for otherwise you'd just pull fresh conductors.
If this is knob and tube... a transition box... get back to us.
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Your OP is so terse that much has to be assumed.
This makes for errant advice, of course.