make two calculated fields in a query
Tbl.fieldName is the name of the field with the string value.
LeftPart: Left(Tbl.fieldName, (InStr(Tbl.fieldName, "__") - 1))
RightPart: Right(Tbl.fieldName, (Len(Tbl.fieldName) - 1) - (InStr(Tbl.fieldName, "__")))
In words:
Left part. Find "__" position, return the left characters up to that position minus 1.
Right Part. Find "__" position number, Find the length of the string, subtract the two, take that many characters from the right side.