Does that mean he is a womb twin survivor? There is a well-known connection between left-handedness and monozygotic ( identical) twinning and is possible that all left-handed people are womb twin survivors.
As for his stammer, there was more to this, in that he also had a slight speech defect over the letter R. You can listen to the original speech that he made before his speech therapy in 1938 on Pathe News.
Watch movie and listen to the speech on the Pathe web site
Left handedness is said to be due to a later splitting of the original zygote, so that the split is into two mirrored halves rather than into two individuals. Conjoined twins often have mirrored organs, including having the heart on the wrong side.