We ran into a situation where a pile o' spearmen were lined up so that the enemy reanimates and the cavalry engaged with them were both directly in front of them and equally distant. For all intents and purposes, the spearmen charged the reanimate flank and the cavalry flank at the same time. We called this OK, and played it that the spearmen just flanked the reanimates and got their charge attack.

Does this sound reasonable to y'all?

I would play that way, too. I see no reason to deny the attack to the Spearmen.

If both were squared up and the spearman hit both simultaneously then I think it right, but if an edge clips your unit a millimeter before your opponents you fail the charge.