I finally have found out the “why” of knitting mittens with 2 thumbs.
The photo is of mittens in a store in Reyjkevik, Iceland, but no one mentioned the “why” .
Today, while looking through an older book in my library, (1985) I ran across a chapter about Icelandic knitting and an article about the 2 thumbed mittens.
It seems that workers thought that they were very practical because they could be turned around if one thumb became wet or worn out.
Sailors’ ones had a cross on them for protection while they were at sea.
The mittens from the western fjords were patterned.