Wifi access point plus additional connection

Dear Volumio community,

I’s like to add the possibility to my Raspberry Pi B+ to be an access point (DHCP server) but additionally connect to a local wifi.
By configuring the interfaces file manually and using 2 wifi usb adaptors, this is no problem and everything works fine.
I followed this instructions ​https://volumio.org/forum/volumio-built-access-point-edimax-7811un-t3524.html but used WLAN1 to be the access point adaptor and not WLAN0. Volumio and me want to be WLAN0 the one that connects to the local wifi.
If I do not enter the wifi configuration and credentials manually to the interfaces file but use the Volumio web interface to configure the wifi connection, my interfaces file is overwritten with just the configuration for WLAN0. So - access point functionality vanishes of course.

How do I tell Volumio to add WLAN0 wifi configuration to the interfaces file and not overwrite it? Or is it possible to have two configuration files?

Appreciate your help and thank you a lot in advance.

P.S.: I do not want to build a wifi bridge between WLAN0 and WLAN1.

The web configuration is the limiting factor here, it can only setup a primary connection and it simply just writes a config file. If there is information in the file it is deleted by design. If this wouldn’t be setup this way people would have a problem when they setup a wrong configuration, if thats persistent it won’t be overwritten thus keeping the faulty configuration.

Basically rewrite the network configuration page (.php) or you could just simply set it once via the webgui and then edit the networking file. But i don’t think this really solves the problem of taking Volumio with you on the go and connect to any (random) WiFi network.

What i would do is see if you can find where the net-config.php page submits to the /etc/network/interfaces file, change the file name to a temp name (setting wlan0 in the next step). In the same folder save the configuration file for the wlan1. Then in the end combine both files and overwrite the /etc/network/interfaces file.