Yes. It is possible to love and be in love w/more than one person at a time. I think if someone is in this position and they evaluate their feelings they'll realize that there are pros and cons to each person they love, or to that relationship (not necessarily sexual).
Most people want to think that NO! humans must only love one person at a time. How if you fall for another it assumes you are disinterested in the current love. These things make it easier to file for divorce, break up, and be angry & hurt. People are emotional and forced to keep so much in and hidden. We are not nicely wrapped in a bow at birth that says you'll grow to only love one person at a time. Ever.
None of this means that I agree w/cheating, polygamy, lying, or demeaning anyone because you "love someone else too".
I am pro-monogamy. Loving more than one person and choosing to emotionally and physically act on it, if you are in a committed relationship with someone, is not right. It's no excuse. It's damaging, dangerous, and harmful. Even if you are single you should not use this "excuse" to play others, men or women.
Communication is the key to many things. So while you can fall in love or be in love w/two people @ the same time, you must choose if you are not in an open relationship. Even those are often damaging.
If you meant simultaneously as in at the same second. Uh, there's some HUGE odds against that one.