I do not see any problems with having one install, and frankly if it is going to be mostly based on the same core it should be. Having two separate installs is really the simplest way, not necessarily the best for the customer. The multi-era problems could be solved by stamping each module you buy (aircraft, maps, vehicles) with an era, or even as granular as a year. Finally, the first module you install decides the era or year in the common DCS World, affecting the UI theme, available units and maps in the mission editor, and filter out unrelated content. Naturally the user would get full control over era settings, so that they may choose to disable era-filtering, being able to put whatever they wish in their missions, or switch between eras.