Good eye. However, just removing the limit of Split make B$ an array with 2 elements

B$ = Split(Split(A$, "-", 1), "_")(1) & "_" & Split(Split(A$, "-", 1), "_")(2)

Better might be
$B = Mid(A$, (Instr(A$, "_") + 1), (Len(A$) - (Instr(A$, "-") + 1)))