I think there's a strong case for the transition rule applying, even though the transaction isn't a typical "merger or acquisition."
The intent of the rule is to provide a coverage transition when different businesses with employees move into or out of a controlled group as a result of corporate transactions. Although not necessarily an acquisition or disposition of an entire business by a third party, the two entities here are nonetheless moving from standalone entities (each a separate "employer") into a controlled group (now combined as one single "employer"). So on its face it doesn't seem to be a clear violation of the spirit of the rule.
As a more stark example, if X was instead owned 99% by Individual C and 1% by an unrelated company, and X redeemed all 99% of C's stock to become a wholly owned subsidiary of the unrelated company, I think you would be hard-pressed to argue that's not a "similar transaction" to C's sale of stock to the unrelated company.
Arguably Corporation X's redemption of C's stock is an "acquisition" of C's stock by X (and C's sale is a "disposition" of the same stock).
Arguably A and B have "acquired" C's stock in the sense that their proportionate ownership of all outstanding shares has increased from 66% to 100%.
Even if not, I think it can be fairly characterized as a "similar transaction" based on the circumstances. If, instead of having X redeem C's stock, C sold his/her shares equally to A and B, that would have the same effect as the redemption and would fit more squarely into the "disposition" and "acquisition" terminology.
I don't have any inside line on the IRS's interpretation, but I read the "similar transaction" provision to mean that, as long as the end result is the same as an acquisition or disposition, the procedural form of the transaction shouldn't change the analysis.