In dual-primary mode, a resource is, at any given time, in the primary role on two cluster nodes. Since concurrent access to the data is thus possible, this mode requires the use of a shared cluster file system that utilizes a distributed lock manager. Examples include GFS and OCFS2.
Deploying DRBD in dual-primary mode is the preferred approach for load-balancing clusters which require concurrent data access from two nodes, eg. virtualization environments with a need for live-migration. This mode is disabled by default, and must be enabled explicitly in DRBD’s configuration file.
See Section 5.8, “Enabling dual-primary mode” for information on enabling dual-primary mode for specific resources.
With current DRBD-9.0 version running in Dual-Primary mode is not recommended (because of lack of testing). In DRBD-9.1 it will be possible to have more than two primaries at the same time.
 Perhaps that feature will be renamed to "Multi Primary" later on, when it actually works ;)