Welome to FT.

We often see when there is only one seat left in a particular fare bucket within economy class such that booking two tickets separately can save you some money. How much $ are we talking about? The person on the cheaper ticket will have the lower upgrade priority (assuming same status) versus someone with a higher fare class if your upgrade does not clear at time of booking.

You cannot place your MileagePlus number in for another person - there is a name/MP match and the system will reject it.

If you book two separate reservations, they cannot be joined together, but they can be cross-referenced in notes in the PNR.