“The two main parties as acknowledged by all historians . . .”

I don’t think all historians would describe the situation as having two main parties. Just to be clear:

34th Congress (1855–57): Republicans held 14 congressional seats; the remaining opposition parties held 157; Whigs had 7 of those.

35th Congress (1857–59): Republicans held 112; the remaining opposition held 18.

The Whig party ceased being a majority party after the 1854 election. The Republican party, which was a third party advocating against slavery, then absorbed many of the former opposition party members to become a major party. Any historian describing the 1860 election where the vote was split (about 40:30:20:10) as a two party system probably has an exceptionally weak background in mathematics. While those parties were the two largest, the other parties still held more than 10% of congressional seats after the election.