Wow. I experimented last night and put a bowl of water on each side of the fireplace. I woke up this morning to one completely empty bowl, another half empty, and, according to the "pinch the skin on the back of the neck" test, two *very* hydrated kitties! This is fantastic news, since one of them - I'd thought - was beginning to show signs of slight dehydration and I thought perhaps he was starting with kidney issues.
Who'dve thought it, something so simple.