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2 uppers- 1 BCG
My thought, I've got no dog in this fight but I'd say go with BCG for each upper and maybe think about a spare BCG or bolt assm. I have 2 spare BCG and at least 2 bolts and spare parts cam pins, LPGs. when I have a parts failure hammer, sear. ect.. change out the part(s) drive on bolts wear in and take a set?? I've changed out buffer detent due to showing wear.
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Makes life easier if the uppers are complete
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I can't see a reason not to have both uppers complete, especially now that the BCM BCG's are readily available.
Of course, I'm not sure how you've resisted the urge to buy/build another lower to have two complete rifles. Complete AR's have a tendency to grow out of spare parts.