To maintain HA there always needs to be a host to fail to - so with a 2 node cluster you will always have the 2 hosts running with DPM -
so host (b) will always open ? is it true ?
yes, DPM on a 2 node HA cluster is not supported since HA is not guaranteed.
http://communities.vmware.com/docs/DOC-11824
On small cluster with few nodes and VMware HA enabled, this feature may make not sense (for example a 2 node cluster with HA enable can not use DPM cause HA will be not guardanteed).
yes on a 2 node cluster DPM will never shut down a host since there always needs to be enough hosts to support an HA event -