That will depend on a few things:
- Do you use it for gateway email
- Do you use it for web messenger
Remember it doesn't load balance by default, you will have to either change their DNS to point it to the other machine, or use round-robin dns, or set this up with a hardware load-balancer (the preferred solution)
So what I would do if you are going to have a cluster permenantly is:
- Setup 2nd UN with same FQDN as 1st
- Install Load-balancer
- Put 2nd UN onto the network
- Set up clustering and allow appropriate time for replication
You can then safely shutdown the 1st to do whatever you want to it, and turn it on without interruption to mail or workflows