If the idea of reincarnation and soul groups exists, then I think it would be possible that a single soul could have a part of itself placed within two bodies...
I recommend: 'The Journey of Souls' by Dr Michael Newton and 'Destiny of Souls' - very interesting case studies.
In terms of 'soul mates' I think that you can potentially be happy with anyone, but I also believe that there could be people out there who are from the same soul group as you - where you have shared many lives together.