Are you using MSCS or pRDMs? If so this wont apply.
With vMotion and DRS(FullyAutomated) enabled and using you would EnterMaintenance Mode on HostA, The VM on HostA moves to HostB via vMotion with no disruption of services. Then HostA can undergo all planned maintenance. When all Planned maintenance on HostA is Completed, HostA is exited MaintenanceMode. HostB enter maintenancemode both VMs on HostB are vMotioned to HostA. Then all maintenance can be completed on HostB without disruption to services. Once HostB is back vmotion the VM back to HostB where it was residing originally.
With regaurds to VMware Updates this whole process is automatic if configured properly.
You need Shared Storage and vMotion as a requirement(at a minimum). Using HA to accomplish this is Not Recommended IMO.
-vFantastic