I think it's perfectly possible to get psychically connected to someone else even to an extreme degree if you focus on him or her. I believe if the other soul is allowing it you can even walk with this other soul and experience what it experiences for a longer time. I should stress that the soul's experience is the same, the individual expression in life might not necessarily be. When you bring up this tresspassing on another one's soul I feel one is walking a thin line, but I don't really believe another soul can influence you to any extent if you do not allow it in.
I even believe it's perfectly possible to be linked to and mutually influenced by another being on a psychic/spiritual level your whole life without you or the other person ever knowing of it or each other. The way I see it, in the closest possible manifestation of this, it's what twin souls are all about, not the romantic notion of it though, which I find puzzling, cause if someone were your twin in the flesh you'd never consider him your perfect someone either.
The big question to me is what do you do with this knowledge of connectedness?