Every twin seedling I have grown out, even when reduced to the strongest one, has turned into a hermie or other wierdo that either did not grow properly or flower properly.
These days any twins get culled by me.
I think the genetic flaw caused by being a twin continues throughout the plants entire life, causing all sorts of weirdness at flowering time.
Keep the biggest sprout if you are keen for an experiment, just cut the smaller one off, however, don't be surprised if it does weird things later in life though.
Although I have only had twin seedlings a few times over the last 35+ years, it seems more common in auto flowers, another reason for me to be anti autos and more justification for deciding to never grow them again!
Life is too short to deal with weird seedlings/genetics!
These days I only grow on 100% normal seedlings and I am rather ruthless when it comes to weirdos or genetic mutants.
But it is up to you what you do.
In week 7 of my diary "cannabis conundrums" you can find pictures of a twin seedling and the hermaphrodite flowers it produced, for further evaluation/reference.
Hope this helps, Organoman.