While I think it's possible to love multiple people at one time, I think it's only possible to be in love with one person at a time. In my opinion, being in love has more to do with the person's whole being, wanting to spend every second of every day with the person and wanting to make him/her happy for no other reason than that you enjoy seeing him/her happy. It's not possible to feel this way with two people at the same time because you cannot want to spend every minute of every day with two people simultaneously. (And if you do, you're not wanting every minute, but rather, half the day and this means there's something else that you clearly want as well, leaving the first 'love' to be alone.) Does that make sense?