Both running here with no problems:
root@n5110:/home/don# apt policy systemd udev
systemd:
Installed: 239-3
Candidate: 239-3
Version table:
*** 239-3 500
500 http://deb.debian.org/debian unstable/main amd64 Packages
100 /var/lib/dpkg/status
238-1.1 500
500 http://packages.siduction.org/fixes unstable/main amd64 Packages
udev:
Installed: 239-3
Candidate: 239-3
Version table:
*** 239-3 500
500 http://deb.debian.org/debian unstable/main amd64 Packages
100 /var/lib/dpkg/status
238-1.1 500
500 http://packages.siduction.org/fixes unstable/main amd64 Packages