Personally, if it was me, I'd just use 2 12's and a 7+12 housing and call it a day, assuming you can get those. Those things get brittle over time anyways (have had the plastic up by where the cutter part of the pins is snap and fly off so many times..), and IDC is frankly just a junky cheap labor-saving method anyways, that's more prone to failure.

Only reason I'd try to save those outdated housings would be if I was going to have to use 3+ connectors to replace one. That's just too much, and too much risk of someone getting confused with what goes where, even if it's labeled.

Well, there went my suggestion. That was the only reason I could see not to use two, and the concern of them not fitting or maintaining proper alignment is entirely valid. Ugh.