OpenWRT: Wifi bridge ("range extender") using both 2.4 GHz and 5 GHz frequency bands

Question

I need to set up an OpenWRT router (TP-Link 3600) in a "range extender" style bridge configuration. Target environment is a normal home network with a Fritzbox connecting to the WAN and no roaming control for wifi clients.

There are several documented ways to do this, but none address the issue of traffic congestion if both the connection to the Fritzbox/WAN and the connection to the clients use the same frequency band. Hence, I am trying to use 5 GHz for the connection between the routers and 2.4 GHz on both routers for the clients.

         +----------+   5 GHz         +----------------+  
WAN ---- | Fritzbox |  .............. | OpenWRT router |  
         +----------+                 +----------------+
            . 2.4 GHz                     . 2.4 GHz
            .                             .
            .                             .
          Clients                       Clients

Following https://juliank.wordpress.com/2014/08/07/configuring-an-openwrt-router-as-a-repeater-for-a-fritzbox-with-working-multicast/ (which is, IMHO, a more concise and readable version of the official OpenWRT relayd recipe), I do not get traffic from the OpenWRT client-side network through to the Fritzbox. I only deviated from that document in that I assigned one of the networks to wifi0 only and the bridge network to wifi1 only.

So, I have two questions about this:

• Does splitting frequency bands as described here even make sense?
• Is there anything that would prevent relayd from bridging between a 5 GHz network (i.e. a network I assign to wifi1) and a 2.4 GHz network (i.e. a network I assign to wifi0)?