Aha. I found a way to get a second set of factions into a requirement line in constructing a building, but it still isn't nearly what I would like. The "or" apparently will keep two things together, so this would work:

buildingalpha requires factions { greek_cities, romans_brutii, } or factions { parthia, } and building_present_min_level hinterland_roads roads

I had been unable to get any second grouping of factions to work. So one set can have no requirements, while the second set has some. But this doesn't do a lot, as I was wanting to have different sets for multiple different factions. If anyone has any idea how to achieve this, please let me know. But the above appears to work. The first two factions have no requirements, and work immediately, but the last one, parthia, has to have roads built first before it will be buildable. But once they are constructed you could build it.