we have BizTalk 2013 Enterprise, currently have a single application server with databases hosted in a another SQL Server, have SSO running together with the application server, no cluster setups. We are running into capacity issues, therefore looking to add another server to join the existing group as 2nd processing servers. Trying to understand the risk it could incur, do we need to plan any outage window. What I would think is just to join and configure the host instances on the 2nd server it should pick up message box naturally, something else I should be aware of? Thanks!!
Your predictions are mostly correct. Newly added BizTalk Host Computers will begin processing as soon as enabled.
Having the Master Secret Server on the other BizTalk server is not a specific problem, but moving the MSS to the SQL Server is generally considered a better solution.
However, for general risk mitigation, you should add the new server during a regular maintenance window or downtime.