Windowless basement is probably the easiest especially if you live in the North! My basement is 48-55F right now depending on the specific location of the room. I'd be worried about an AC going out or a power outage, but I do know window units are cheap and the newer ones are pretty powerful. A closet is often a few degrees cooler if it has a door since heat is usually not blowing into it. I imagine frogs will like it warmer than salamanders, so I am not convinced I would even attempt to use the same room for both. My snakes are in a different room altogether than the salamander room I am building in the basement although I'll use neighboring space to cycle them so i don't need to cool the snake room any longer.
