I think if you were already dating the one twin, you'd have connected with that twin enough already to avoid a situation where you'd become attracted to the other twin - you would have developed the relationship to associate your physical attraction to the twin you're dating with their personality and all the other little things that become part of that connection with someone - therefore when you hear about and meet the other twin, these additional qualities would not be there and you would not find that twin attractive.
If you met both twins at the same time, you might initially be physically attracted to both, but often even in those early encounters you'd probably make distinctions between the two based on the physical appearance and early dialogue - different clothes, body language etc - I've never dated a twin, but my experiences of knowing twins is that they have always had different enough personalities for me to connect with one more than the other - so I think I'd only ever be able to romantically connect with one
There is a possibility that as time progresses and you have gotten to know both that you discover you're actually incompatible with twin 1 and are much more compatible with twin 2, but this would surely be an lengthy process much further down the line and something that would not be easy for anyone concerned.
I think meeting and getting to know twins that you're initially physically attracted to probably illustrates how someone's personality and indescribable 'ways' is what ultimately makes you really fall for someone and that the initial physical attraction has much more meaning to it once all those other factors of falling for someone have combined.
TTG