Occasionally during lives if two souls bond close enough and die in a short period of time then their souls will cling to one another. While the souls themselves couldn't tell they were together, it would take a great force to separate them. It's rare, but when one of the two souls get found for Serra then the two souls linked together will be born as twins. The two often being nearly inseparable, their connection basically being magical. So much so that some researchers are looking for a pair of willing twins to get a better understanding of how the bond actually works. There are other interesting customs in terms of twins, polygamy with twins is fine so they aren't as separated, and the twins are guaranteed to be identical.
Regardless your previous life you were lucky and were blessed with someone you could truly love, but were unfortunate enough to fall under the worst misfortune with the love of your life. Still the two of you were still together, and after developing into a baby with your lover the two of you were born as a set of twin...