You can find a Durantium and an Elerium within single starbase effect. I'm doing it now. A Large map with default settings should have 1-3 places where this is possible.
My best single mining starbase had a Durantium, an Elerium, and two Antimatter, and with the +2 hex range increase I would have gotten a 2nd Durantium. Yes, it does placidly mine all 4 every turn, with 4 wavy lines on the map.