WLAN only for one wireless with OpenWrt 19.07 rc2

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 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.

Is it a bug? Any suggestions?

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"

Since I am relatively a new OpenWRT user, could you please point me to a right direction, a link for reference perhaps?

Not sure if I will need to install delayd package for my purpose.

No relayd!

in the Web Gui:
Network -> Interfaces -> Edit -> Physical Settings -> Select "Bridge interfaces"

Reset settings to defaults and see if the issue persists. Make sure you backup your config prior to doing so.