No, you can’t just do it for 1 segment. You can do it for ‘credit’ purposes, but only be removing it completely and the selectively retro-crediting per segment. What you want to do is not going to be easy - you want to double-dip on both benefits. Now, if you don’t mind where your miles go, then showing your AC25K credentials at check-in will be your best bet to get your bags. The UA E+ should stick once assigned and not throw you out. Once at EWR you can get your UA# back in for lounge access etc. - where this will get credited is going to be YMMV with AC/UA and multiple FQTV#s in play. Your best bet is the AC agent simply waiving the bag fee with 25k and not changing the FQTV# - whether they will do that is pretty uncertain and perhaps not that likely. So pick your poison