The dems tried that in Louisiana when they were the dominant party. Step one - agree on a single republican candidate. Step 2 - get a bunch of folks to split the dem vote. A radical vegan environmentalist against a black militant against a feminist - throw in a couple of flavors of Mexican. It’s a bit expensive on the front end, but it makes sure the republican is the leader going into the runoff. And by that point, the trailing dems have been demonized by the leader of their ilk to the point they won’t back the leading dem.
Eventually, the money was pulled, and the republicans won.
Not a bad idea, but the Republicans out here also have an ego and would force big fights among many.
They need to grow up.
Your idea is 100% great though IMO. Good thinking.