there is also the possibility that the tickets were tied together so when neither one of you boarded with your group, it flagged the system. Once you didn't respond to the call out, they gave both seats away after your boarding group. I have seen in some cases, one partner will board first to put up all the carry on items and the other partner boards later with the baby. If your tickets are tied together, then they are less likely to give away your seats.
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