"While striking bottom is done by heaving the lead, it usually has only one sinker on the line, a tapered thing like a sash-weight, with a hollow cup indented on the bottom for the wax plug used to collect bottom sample. I have never seen or heard of a two-lead method, and I think this is an invention born of misunderstanding."

Amos' explanation sounds sound to me!
It may be that the operation was carried out twice! The second time to confirm the first reading!
P