Falling in love is not an exclusive and unique emotion that never repeats itself. But that being said, it's not really easy to fall in love with two people at the same time.
It's always easier to fall for someone else when you're already in a later stage of love with your present partner. When you're in two different stages of love, the emotions you experience don't contradict each other. And it's two unique experiences that come together at once.
You may be passionately in love for the first few months with your partner, but mature love is very different from the passionate fiery love you experience at the beginning of a relationship. So when you experience the same fiery passion with someone else, it's easy to experience both stages of love at once.