Well you could try either way, but I think set the sidechain in the Lead to listen to the Doubles. That way, if the doubles get louder, so will the Lead, if the go quieter so will the Lead. So it always sits on top of them.
But you can limit it to just getting louder if they need to, by only using the “positive” slider. It might take of experimenting to get the right balance. It will be quite important to get the threshold of the Lead plugin sitting in just the right spot to keep it balanced. I suggest starting st a point in the song where you thing the vocals are perfectly in balance, cycle that region and set the threshold so that it’s sitting as close to no gain as much as possible. Set the threshold before deciding how much boost or cut it may require.
At least, that’s what I suggest trying at first,