Today I was asked by a colleague whether we can run User Profile Service instance and Profile Synchronization service on more than one server in the farm and the answer is Yes!
Q1) Assume that you have started the UPA instance on your app server (e.g. APP1) and the Profile Synchronization job, what happens if APP1 goes down?
A) Like other SharePoint 2010 service applications, UPA can be load balanced so you can start the User Profile service instance on a second app server (e.g. APP2). If APP1 goes down APP2 can handle the request and there wont be any outage.
Q2) What about Profile Synchronization service?
A) You can start the Profile Synchronization service on APP2.
Q3) What about Sync connections and other related configuration, do we still have access or should we start from scratch?
A) All those setting should be available after you successfully started the Profile Synchronization service on APP2, as all those settings are stored in Configuration database.
Q4) What happens if I already started the Profile Synchronization service on APP1 and by mistake start the service on APP2?
A) Provisioning process will stop the Synchronization service on APP1 and start it on APP2.
Q5) Can I run more than one Profile Synchronization service in the farm.
A) If you have more than one UPA, you can start the Profile Synchronization service on separate servers, i.e. running the sync service on APP1 for UPA1 and APP2 for UPA2 simultaneously.