What if Kentucky went to the Union's side early and Tennesee seeing itself truly threatened early declares for the Union as well.
You would have to completely do away with the Tennessee government at the time. As well as the internal strife which was already exploding before Tennessee seceeded. Before Fort Sumter a vote was held which gave a 4 to 1 majority to no leave the Union. After Fort Sumter, it was the exact opposite. Franklin County Tennessee actually threatened to leave the state and join Alabama if Tennessee did not leave the Union.
I can go along with Kentucky declaring for the Union, but Tennessee is a bit tricky. But regardless, if both states went with the Union, I'd doubt the war would even last until 1864. It could well be over by 1863 - especially if the Union concentrates its efforts out west
Why would KY declare for the Union early? The best way to have this would be that Magoffin dies before the war and the state House put a governor in place, though I think you'd still get a shadow/secessionist government at Bowling Green or Somerset.