How to make NavMeshAgents traverse two possible paths

I have NavMeshAgents in a simple orthogonal map with two paths of the same length. (I am creating a Tower Defense game.) The NavMeshAgents always travel in one of the two paths. How do I make half of the agents go the other path?
If more detail is needed, tell me.

I don’t think you can querry for multiple paths, but you can define costs in areas and randomize how agents are able to walk them: