You've heard the term 'multi-tasking', yes? Technically, our brains actually process lots of things quickly, but it's really one thing at a time; just very very fast. So, 'at the same time' is a bit of a trick.
That said, I believe love is a verb, not a noun to describe a feeling. You can easily make the decision to take loving actions towards more than one person. and you can have boatloads of feelings for many, many people.
Love is that condition in which the happiness of another person is essential to your own...
Robert A. Heinlein
Me: female, bi, (formerly hinge of a vee)
with FirstBoyFriend (FBF)(moderately long-distance)
and no longer with CurrentBoyFriend (CBF)(who lives in the apartment building next door)