I think you can be in a relationship with someone who you think you're in love with but falling for someone else and realising you were never really in love (or have fell out of love) with the first person is more likely to happen.
If someone is actually in love with two people, then they have obviously let a situation happen that shouldnt have. Love takes time to grow, it doesn't happen immediately.
I actually agree with you Lore, that if you fall in love with someone eles then you have either fallen out of love with the first person or weren't in love with them in the first place. Emotions are so tricky though...
They certainly are hunni, been there done that etc etc etc..
however, I knew I wasn't in love for a long time but until I fell in love it was quite a surprise to realise I was never in love (with the first person) in the first place, if that makes any sense at all?!