Can two devices on different WiFi bands connect to each other through the ASUS RT-AC66U?

Wireless interface really looks like wired interface once connected. I mean when you're done with authentication, your wireless connection will emulates a wired connection, and you should "see" the network as you were connected with a wire on the router.

In most wireless access points or routers the default configuration is to make it work for most users, so to let communicates clients with clients.

So your ASUS RT-AC66U should let client communicate with each other.

On this router, there is an option to prevent client to client communication connected on the same wireless interface, named Set AP isolated in the Wireless>Professional tab (reference).

Yes... The key to remember here, at least in this instance, is that the WiFi connection is just layers 1 & 2 connection to the network regardless of the band involved, meaning in layman's terms it is taking the place of a wired Ethernet connection. To the rest of the devices in the network, it is just another node. The fact it is wired or wireless is not relevant. The actual IP connection at layer 3 and all higher layers are identical to a wired client.

Unless some other factor is involved (like AP Isolation), all nodes, both wired and wireless, are on the same network, meaning the rules of them communicating with each other are the same regardless of the underlying connection.

Yes that will work, since they're both connecting to the same IP subnet. (I assume) The wifi bands are comparable to having two different types of cables, for instance one being a cat.5e and another being cat.6 - both devices will connect to the same ip subnet but with different connection speeds.