My are also 2x4s. When they first started roosting I gave them the option of a 2x4 or 2x2. They all crowded on the lower 2x4. They now have 2 2x4s and seem happy.A picture would help.
My roosts are 2x4s so that they can fully sit on their feet in the winter months to help prevent frostbite, their feathers keep their feet warm.