Occasionally, in states that allow two parties to jointly nominate the same presidential candidate, the presidential elector candidates of one party are different that the presidential elector candidates of the other party. When this happens, the vote for the two competing slates can’t be added together.
This year in New York, the Independence Party and the Libertarian Party both nominated Gary Johnson, but the two slates of electors are different people. If Johnson were close to carrying New York, having his vote split into two separate slates would ruin his chances of electing either slate. But this is only a theoretical problem, because it seems unlikely in the extreme that Johnson could carry New York even if both slates were the same names.
This problem existed for Robert La Follette in 1924 in Montana and Oklahoma. He was nominated by three parties in Montana, and two parties in Oklahoma, but the parties didn’t submit the same electors. La Follette came fairly close to carrying Montana. He polled 65,876 votes, for second place behind Calvin Coolidge with 74,138. But if La Follette had received the most votes, he still probably would not have won any electoral votes.
It is too late for the Libertarian candidates for presidential elector to withdraw and be replaced by Independence Party electors. The deadline for that was September 5. The New York Independence Party hadn’t even nominated anyone for president at that point. See this Politico story. Thanks to several people for the link.