I’m trying during the installation from MacOS High Sierra to separate the system and user data. I splitted the SSD with Disk Utility, but I can’t mount the second APFS Volume to /Users with the Disk Utility. Is that possible at this stage?
Not with Disk Utility (can't specify custom mount points with Disk Utility) but the macOS installer environment has Terminal. With Terminal, you can run:
mount_apfs /dev/disk1s3 /Users
Replace disk1s3 with the identifier of the volume to mount there.
This should answer your question about mounting during installation, but after a reboot your disk will be unmounted and /Users unavailable. To permanently set a mount point, you'll have to mess with fstab.