Hi Abhishek,
Generally, if primary server is down, it will find the secondary server to authenticate and vice versa. As far as I know, primary server will load balance. It depends on the configuration of the HLB. By default, in ADFS server, the database is Windows Internal Database which store user data. So, if you use Windows Internal Database, the data will not be synced between primary ADFS and secondary ADFS. They will not create communication unless you use the SQL database which locate in the same place for both servers. Regarding “the dc communicates with our ADFS through HLB”, as I mentioned, it also depends on the configuration of HLB.
For Outlook disconnectivity issue, we need to case by case to check the issue. Based on my experience, it will not cause the memory issue(I am not sure if HLB impact on it and I have not encountered the similar cases). As for the case, we need to confirm if all users try to authenticate with ADFS server and cause the memory issue. Also, we can check the log on ADFS server (ADFS log, IIS log) to find the root cause of the issue.
Thanks,
Johnny Zhang