Success! Both monitors works!
I can´t explain what the problem was, but the following steps fixed the issues:
- format SD card (with SD Card Formatter)
- install/update the current version (5.8.88) of WTware on PC
- start WTware Center
- create new Boot SD
- put in the Boot SD in the Raspberry Pi > power on
- get correct configuration to the Raspberry Pi while using WTware Center
- reboot RPi4 > works
No special settings for second monitor needed, config.txt left as created.
Thanks for your help!