Setup a Wifi-Hotspot with no Ethernet but two wireless adapters

I want to setup a headless Raspberry 3 A+ (no ethernet) as a WiFi hotspot.
The device runs its own web server and application, it should not necessarily provide internet access when connected to the hotspot.
It has two wireless adpters, the internal one and a RTL8188EUS Wireless adapter plugged in the USB port.

The goal is to use the device off-grid as a web server for a dedicated application.

The HotSpot should use the external USB adapter, since I use the internal one for SSH when connected to my local WiFi network.

How do I configure the external adapter as a hotspot manually? The dietpi script unfortunately requires an ethernet connection but the A+ does not have one.

Thanks for any advice!

Yes, unfortunately a limitation of our install script. Theoretically you could do this with 2 WiFi adapter, but you would need to configure everything manually. You could use our install steps as idea how it could be. You just need to replace interface names with yours.

Thank you for the answer, I managed to install and configure hostapd. It starts without error.
However, the AP is not visible.

ip -a shows:

wlan1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000
    link/ether b8:27:eb:8f:5b:32 brd ff:ff:ff:ff:ff:ff
    inet brd scope global wlan1
       valid_lft forever preferred_lft forever

…so it is still DOWN, but LED on the adapter shows some kind of activity.

Any ideas?

Compare with the steps I wrote down here: DietPi as WiFi router? - #33 by MichaIng

In your case, try

ifup --force wlan1
systemctl restart hostapd

I managed to get the hotspot running.
The issue was channel=0 in hostapd.conf which gave an error along with unconfigured dhcp server.
That works now.

Now I have problems with interference with bluetooth, but this is not part of this topic.