If your intent is to get the last name (aka second), then you can do this
name = ['a', 'a b', 'a b c']
[i.split(" ")[-1] for i in name]
['a', 'b', 'c']
But if you want to concatenate them, then you can do this
[" ".join(i.split(" ")[:2]) for i in name]
['a', 'a b', 'a b']
So the field calculator expression is one of the above like
" ".join(!name!.split(" ")[:2])
... sort of retired...