In a way, it's a nested roundtrip. Because of your planned stopovers and the way UA's hubs are aligned, the lowest fare combination turns out to be one roundtrip nested in another, even though you are only connecting through ORD. This happens occasionally, and it's perfectly legal. You might run into trouble with CS agents during irrops because the fare rules are more convoluted than what they normally see. Also, should you need to make changes, you might be looking at two separate penalty fees and perhaps also repricing. Still, go with it if you are comfortable with this itinerary. But proceed with caution!