In article ,
says...
I have a young black walnut tree growing in my garden in western
Massachusetts, probably 6-9 years old (just guessing). There's the main
trunk, about 6" in diameter, and then two small 'trunks,' about 2" in
diameter, attached to the main trunk at ground level. Should I prune the
small trunks out?

At least. To me, the judgement would be whether to keep it at all.
Black Walnut has a lot of genetic variation in branching. Early
and low branching are usually deemed inferior traits. This may be
based partly on desirability for timber, but large crotches are also
weak points where breakage and rot happen. I'm guessing since it
has already branched so early and low, it may continue to behave
that way. But I would consider the form of the dominant trunk.
If it looks like it wants to go up straight, it may yet make a
decent tree. If it is already growing more two and three-way
crotches, what you have is a large bush, which I might whack.