Remove link in /etc/xen/auto to avoid starting machine twice by accident.

Temporarily allow dual-primaries

drbdadm net-options --protocol=C --allow-two-primaries <resource>

And to go back to normal

drbdadm net-options --protocol=C --allow-two-primaries=no <resource>

To check the status of dual-primaries

drbdsetup show <resource>