I agree with Jeff. You should be ok. But I think you need to define what the two Unity servers are doing. Adding another Exchange server to an existing Exchange Site is simple to do. Looks like another server in the Org. Adding a DC does not harm the existing AD. But if you are adding these servers from over a slow WAN link, think about your design for Sites and Services and Exchange Routing groups. If not designed correctly, it will hinder you performance.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.