Sure they could have !
You are just combining the actual votes received.
You are not looking at what the effects of a combined and unified campaign would have had on the votes and the election outcome.
They lost a massive amount of campaign time and good will just by having a failed convention!
Both factions had to hold a second convention to select their nominees.
That would sap a lot of energy, money and time out of their respective campaigns.
Look at the states where it was only Lincoln vs Douglas and the states where it was Lincoln vs. Douglas, Breckenridge, and Bell and the total Democrat vote isn't all that different. It's not like the Bell or Breckenridge supporters were voting for Lincoln when their man wasn't on the ballot.