What's your perspective?

So the other day I ran into an argument. The argument was about whether someone can love more than two people at the same time, I'm mean deeply love not just friends or a crush. It started with the advice given " If your in love with two people always go with the second because if you truly loved the first then you wouldn't love the second".  I think that its possible to fall out of love if you don't constantly work at the relationship because lets face it people we are a society that gets bored easly. We're constantly upgradeing, tossing out the old to bring in the new, and sometimes we do the same in relationships. But I also believe that if you're truly in love with someone it will work out as long as you try. So please tell me what you think about this "If your in love with two people always go with the second because if you truly loved the first then you wouldn't love the second". Do you agree or disagree?