The disadvantage of having a single Connection Server to support internal and external users is that internal users will then have their desktop traffic (PCoIP) gatewayed through the Connection Server. For internal users, this is not as efficient as doing PCoIP directly between View Client and virtual desktop.
By installing a second Connection Server (a replica instance) you get the best of both worlds. Remote users (using one of teh Connection Servers) have their PCoIP gatewayed through the Connection Server. Internal users perform PCoIP directly giving a better user experience.
This setup has other benefits too. You can use tagging on pools to ensure that some pools are only available to internal users etc. You can't do this if you try to do everything through a single Connection Server.
These are the reasons why we recommend having dedicated Connection Server(s) for the internal and external case.
Mark