Ceph

http://ceph.com/docs/master/rados/deployment/ceph-deploy-mon/

You can establish a monitor quorum with only one monitor; however, you can not determine a majority with two monitors. A majority of monitors must be counted as such: 1:1, 2:3, 3:4, 3:5, 4:6, etc.

You can establish a quorum with two monitors, it's just not good deployment practices because it doesn't provide any extra durability over one monitor. There's enough confusion on this topic in the community that we need to be very careful about our own docs (should probably audit for similar references or confusing statements).