By default quorum is 50% + 1Vote and with 2 nodes to have quorum you need 2 votes.
Yet in a two node cluster - you want to have quorum even with 1 node, so there is a special option "two_node:1" which tells that (we need only 1 vote to survive) to pacemaker.

So ,in your case you need the "two_node:1" and set a proper stonith device (no matter sbd or other). Otherwise - this will never be a cluster,as it will be susceptible to split-brains.
If you add a 3rd node via "ha-cluster-join" that flag (two_node) is automatically removed and should be added when you use "ha-cluster-remove" to reduce from 3 nodes to 2.