I actually think that once the DH-4 is released, we're pretty good in the two seater department. I'd put two more in, for the early war scenarios. We really don't have any observation types for the DH-2/Fokker Eindecker period.
French types of the time incuded Voisons and Farmans. Don't know if anyone would want to be up in one of those. Two good choices, only in my opinion of course, would be the BE-2 and the Aviatik.
"From our orbital vantage point, we observe an earth without borders, full of peace, beauty and magnificence, and we pray that humanity as a whole can imagine a borderless world as we see it, and strive to live as one in peace."
Astronaut William C. McCool RIP, January 29, 2003 - Space Shuttle Columbia