When Are Two Signal Flags Needed?

The painting shown above (click on to see larger version), is from the Signal Corps Museum at Ft. Gordon. While certainly depicting a scene from the Spanish American War era given the uniforms and ordinance alone, Brown's book page 309 explains the use of two flags of different color to communicate from the same point to different stations. Two flags may also have been used to offset atmospheric differences. No evidence exists to support such use, however.