Is it possible to use the carambola2 as an access point with certain SSID but also as a WLAN client for another SSID?

Yes you can, but it didn't work very well when I tried. Things like frequent disconnections etc.

Well..it shouldn't last long this type of mode...

Just for a device that acts first as an AP for initial configuration, then the user can browse for available APs, and after selecting, it should reboot and act as a client only then...

Are there any configuration snippets available for this dual client/AP mode?

You do not need both modes *together* in that case. At boot, you have an AP running, user connects to it, opens web thing on phone, you show a list of APs you detect using 'iw dev wlan0 scan' maybe, then and put those APs in a drop down list in web page(also need to parse iw output and decide what encryption value to use(none,psk,psk2,wep etc used in /etc/config/wireless). Once, selected you can diable this wifi-iface, and configure a new one and enable it, and restart wifi.