If you are on a single reservation, then prior to about 24 hours before your departure (from BOS), you would clear together or not at all. At that time, if you haven’t been upgraded yet, the system will split you automatically and then you could clear separately. If you are on separate reservations, you will clear separately, and there’s no way to change that.

If you’re a gambler, you could try waiting and hoping to see R2 on the outbound, which would allow you to book both tickets at the cheaper price. Of course, there’s a very real chance that the price goes up rather than down. You have a second problem — the return is more heavily loaded in Premium Plus than the outbound, and currently inventory is at O9 A2 R0. That means you’re trying to book what are currently the last two seats in the A fare bucket. If you book one ticket, the system may zero out the A bucket. I’m not saying it’s likely, but it is possible — inventory is dynamic, but UA does seem to update it when it sells a seat, and while the most likely outcome for you buying one seat is that it goes from A2 to A1, it is not the only possibility.