If you're using stock Asus firmware or Merlin, you can only use both the 5ghz and 2.4 ghz radios if you're running in Repeater Mode. When you set up Repeater Mode, you select the router that your Repeater will connect to, and in the process, select whether you want to repeat a 5ghz or 2.4ghz channel. The LAN ports also work when in Repeater Mode, so that any device connected via a LAN to the Repeater is essentially using whatever bandwidth and higher throughput than you'd be able to achieve by receiving/sending via that 5ghz channel (theoretically higher than if you were to use a 2.4gz channel, provided you're close enough to the source router).
Both the 2.4ghz and 5ghz radios on the Repeater also work to allow you to connect clients to them wirelessly. Bandwidth will not be as good (roughly halved) compared to if you were running the AC66 in Access Point Mode, but if you can't run a direct LAN connection to the AC66, then your only choices are either Repeater Mode or Media Bridge.
Media Bridge Mode will NOT do what you want, because in Media Bridge, you select which radio you want to use to connect to the source signal (5ghz or 2.4ghz) and the other radio is simply turned off. In other words, in Media Bridge Mode, you can only use the LAN ports to connect devices, and the wireless connection (receive and backhaul) to the source router is handled by one radio, but not both.
This is the current state of affairs as to what you can do on an AC66U using current Asus and/or Merlin firmware. If you can get your hands on some early Asus firmware (i.e., from around the time the AC66U was first introduced), and set up Media Bridge on that older firmware, that allows the unused radio to also be set up to connect client devices wirelessly. In other words, you'd use 5ghz as the send/receive radio to the source router, and use the 2.4ghz to connect remote clients. You can't do that though with current FW.
Others say you can do this with Open DDWRT, but I have no experience with that. I have two AC66U's that I use along with an AC3200 in my home, with one AC66U set up as a Media Bridge (that I use to connect home theater equipment to in a media room), and the other is set up as a Repeater (to which I have clients connected to both its 2.4 and 5ghz radios and to the LAN ports). Both work fine and both serve the intended purpose.
But if I was able to run Cat5 or 6 to these devices, obviously, I would and then I'd run them both in AP mode, because that's clearly the best and cleanest way to get the fastest connections and the best throughput possible. Sadly though, I cannot run cabling, but I've not been disappointed with this current setup. Speed and throughput are more than adequate.
I am thinking about using MOCA to connect both devices since I have coax connections for cable TV close by both of my AC66's, but I've not tried that yet.