I've done a pace line where it was basically backwards from normal. Last person pulled left and moved ahead. Next last person would immediately fall behind that guy and go with him. Basically, two people at all times were pulling with one fresh the other not. It was for high speed only, not for energy efficiency.
Sounds like you already have this all worked out. I wouldn't second guess yourself. The IM guys already have the mileage under them (especially if you consider the hours required to do the bike/run without benefit of drafting).
Just stick with your plan. You can always ramp it up at mile 150 if y'all are feeling spunky. Or, more likely, whoever has the most energy at the end will just spend more time up front.
I've technically only done one double, but it was solo and all climbing. But it's not on Strava, so it doesn't count (Garmin failed)