If you are talking about different parts of your life, then yes,
it could happen. If you are talking the same time at a particular
phrase of your life - before you claim your love to them, look at
yourself. Usually 75% of them falls under the category of
indecisive, psychological defense, etc, etc - which gives the
magical effect of loving two people at the same time, when by
actual theory and in practical - love is mutually exclusive. The
other 25% is truely a tricky love lesson you may have to go through
- probably karmic by nature. This 25% could happen when you are
already in a relationship - then the mystical one walks along.

Cupid is insane to the point where, for you to grow, there must be
lessons which involve suffering. (After you are hurt, you grow
stronger tomorrow) And we as a free-will user, could reduce them
greatly if we are evolved and see through what lies ahead.
P.S: Don't claim to love two people at the same time before looking
and reflecting upon yourself. Your external factor often has no
problem at all - its all about you.
Cheers