Running DRBD in dual primary mode is pretty much the same thing as accessing SAN storage from two server nodes. All precautions that are valid for SAN are valid for dual primary DRBD. The short answer is yes you can do it, but you should also heed SwenW's warning - never mount the filesystem from two nodes at the same time.
At my company we are actually using dual primary for lots of our clusters (with ext4 or ext3 file system), but it comes with a price.
To give you some real-life background, we used to have SAN based clusters and in that case you of course always see the block device from both sides (similar to how DRBD in dual-primary node works). We also used HP Serviceguard to manage the clusters. Serviceguard uses LVM tags (
vgchange --addtag and
vgchange --deltag) for disk locking, and it also does very extensive tests to protect you from a split-brain situation. In short, if you let Serviceguard manage those disks, it will never make the mistake so you can use whatever filesystem you want (in our case, ext3, lately even ext4). It also requires that the disks are visible from both nodes (it is part of the cluster validation - it makes sure the devices are available on all nodes).
After we retired that expensive hardware (the SAN) we kept the expensive software (Serviceguard) around and so we had to use DRBD in a similar setup with block devices visible from all nodes -- i.e. dual-primary.
Of course, nothing can stop you from mounting the wrong disk by mistake and then you are toast. But that is a warning that is valid when you access a SAN disk from multiple nodes anyway.