You don't have two CPUs doing the routing. Only the one functioning as a router is routing, the other one is merly doing switching and wireless duties.
Its roughly equivelent to settings things up so that the AC3200 Asus router is not using "smart connect". Your devices will be deciding which base station/band to connect to, unless you choose to setup different SSIDs for each one.
Frankly its a pretty crudy setup IMHO.
Then again, I also am not a big fan of the AC3200 routers. In theory, they could be good, but in practice it seems like they've got some issues going on. What you'd likely be better off doing is setting up a seperate 5GHz SSID for 11n devices and then one for 11ac devices.
Or you can recognize that in a lot of cases, you aren't going to have a whole slew of devices being highly active at one time. The performance penalty for something like an iPad 2 streaming netflix on your wireless network is pretty minor.
My wife spins up Netflix on hers and the performance from my N600 AP (if it happens that my laptop and her tablet both happen to be on the same band at the same time and connect to my AP) drops from ~25MB/sec if I am in the same room with my laptop as the AP, down to around 21MB/sec.
Now, sure, if she is transfering a file to her iPad 2 from the server or something, I see a much bigger hit, but even then I might still might be managing 14-16MB/sec to the laptop while her iPad 2 is getting ~3MB/sec to/from the server (that's about the most it can manage for SMB transfers, even without other wireless devices active).
There is a bigger hit if it is my router which is 11ac. Then if she is streaming and happens to be on the same band (5GHz lets say) my performance drops from 55-60MB/sec down to ~50MB/sec and if she is doing a file transfer it might get knocked down further to 42-45MB/sec.
Yes, I wouldn't mind if it got band steered away and didn't interfere at ALL, but the performance penalty is relatively minor.
Even in a "worst case" for my household which has both my son's streaming netflix to their android tablets and my wife pulling pictures off the server or something I've never seen 5GHz 11ac (if I am still same room or ajoining) drop below 40MB/sec and on my N600 router I've never seen it drop below 13MB/sec. In general numbers are MUCH higher, in part because it is rare that all of their devices happen to actually be on 5GHz. Often times looking at device assignment I'll see maybe 2-3 other devices on 5GHz with my laptop, and those are often our phones, occasionally it is one of the tablets (or my tablet), the others are on 2.4GHz...and there often are not more than 2-4 "active" devices on wireless (streaming/file transfers, heavy webpage traffic) and odds are good, there might only be one other device on the same band as my laptop that is active.
Then add up that there is an AP serving one side of my house and the router on the other and the odds go up that one or more devices, even if ON the same band, also happen to be connected to a different base station on a different channel....
Anyway, a long way of saying I feel a lot like dual radio (for the same channel) routers are a bit of a solution looking for a problem. You are MUCH better off trying to divide things up by zones and cover each zone with its own access point/router as well as rely on a bit of spread across the bands, instead of having all wireless devices on just one band. I don't seperate my network with distinct SSIDs and I find that they generally do a decent job of connecting to different bands.
Sure, I look at band assignment sometimes (the rare times I do, generally only if I am specifically testing something) and I can occasionally see 6 wireless devices on one band and a single wireless device on the other band, but most of the time I'll see 2-3 on one band and 3-4 on the other band.