IME UA never has Saver (either as part of a *A award or a pure UA Saver award) J inventory for NRT-BKK on their metal; I guess they sprang free a seat for you based on the circumstances surrounding your reservation issues? Again, ime, you are far more likely to see a J seat open on NH, than on UA. I'd stand pat for now, and even consider flying on separate flights if this was the only option for both to be in J, and keep checking with UA re: switching one UA J seat to NH.