After many trials and errors, I have 3.0.0.4.386.41793 running stable.
I have two RT-AC68U, firmware 3.0.0.4.386.41634, running as nodes.
Knock on wood, it's working.
But, if I run Wifi Analyzer, I see the three signals for the main network but only one for the guest. Either there is a setting that I'm missing or it is not fully flushed out yet. I'm hoping future releases will fix this.