Multiple SQL Server instance install question for AlwaysOn I have a 2 node WSFC cluster with SQL Server 2014 installed on each for AlwaysOn.

This is working fine. My question is can I install another instance of SQL Server 2014 on these nodes for another AlwaysOn configuration. 

I have a requirement that Skype for Business (SFB) has to be on its own instance to be supported and can't be installed on the existing AlwaysOn already up and running for some other DBs.
Unless you can point to the direct requirement you are referring to, I do not see any requirement in Skype for Business (SFB?) that states it has to be on a dedicated instance. 

I think what you may be referring to is the database needs to be in a dedicated Availability Group. If that is the case you can create another AG on the same instance and cluster to support your database for SFB. A given SQL Server instance can support multiple AGs and each one can have their own listener.