Since I upgraded OpenWRT on my Archer C7 v2 to 19.07 rc2, I found that either wlan0 or wlan1 can be associated with WLAN interface, just it can't be associated to both like it used to.
To make it more specific, I intend to let WiFi 2.4ghz and 5ghz connected to outside through WLAN, like a guest network, which has 192.168.3.1/24 as ip ranges for devices connected to WiFi. Formerly I can share one interface 'guest' to devices connected through both 2.4ghz and 5ghz. It seems not work anymore with 19.07 rc2.
In OpenWRT/Network/Interfaces, the guest interface's 'Physical Settings/Interface', only one association could be selected.
I remember it usually work, even when I am using trunk nightly builds.
You have to make it a bridge in LUCI to be able to add more than one interface:
Network -> Interfaces -> Edit -> Physical Settings -> Select "Bridge interfaces"