I haven't done the same exact thing, but have often run bikes beyond derailleur take up capacity.

Typically this would be a triple front with a granny, but the details don't matter. Simply put, you size the chain, looping the big/big combination, and accept that the small/smaller combos will have chain sag (slack). It doesn't matter since you won't use those combinations anyway, but no harm will come should you shift into them.