@FurfangosFrigyes
a second bridge is usually not advised unless there is a really complex setup.
Guest network is not considered being part of that category.
@pakikje
It seems to be in help pages:
https://help.mikrotik.com/docs/display/ROS/Quick+Set
Guest network: Useful for house guests who don't need to know your main WiFi password. Set a separate password for them in this option. Important! Guest users will not be able to access other devices in your LAN and other guest devices. This mode enabled Bridge filters to prevent this.
Bridge filters are being used to do the isolation. Guest to LAN and Guest to Guest should be blocked. Only Guest to WAN will be possible if I understand it correctly.