What is the practical Application?
You really want two or three users sitting at different displays on the SAME computer? Why can't they each be at their own computer and be sharing the data?
Do these need to be full-blown Mac OS X or will Terminal Windows (unix shell) suffice?
What are we trying to optimize for? Computer cost? File Sharing?
This is a User-to-User forum. The time scale for multiple good, thoughtful responses is about a day, not a minute.
I'm just trying to save my money. Us kiwis, unlike most other people in the world, are NOT rolling in dough, and can't really branch out 9000 odd dollars just so each person gets a single computer. And no, iMacs are out of the question, as they are far too expensive. I just wanted to know if I could Let my employees use a computer that is cost efficient. Plus, I think you can choose not to have mirroring and make each display individual.
Mac OS X is not a Time-Sharing system. Mac OS X does not easily support multiple simultaneous Users with a full Graphical User Interface.
You can have multple accounts on a Mac so that several Users can have complete control of everything after they log in. That will allow them to share the computer, but not at the same time.
Using a central Mac OS X Server, you can store ALL User files on the Server and set up a computer Cluster where any User can log in at any computer and have Instant access to their files. This is used in schools to provide a pool of computers -- where any computer from the pool is instantly available to any student.
Multiple-display support on Macs is used to build an larger Extended Desktop for the currently logged-in User, like this:
If high cost is your main concern, look at the Mac Mini. It is like a laptop computer than is not portable, but starts at US$599. Bring your own display or use a Hi-Def TV; bring your own keyboard.
Also, less-expensive Refurbished Macs from Apple are in pristine like-new condition and are available for sale under the same warranty as new.
Sorry, one user on the computer at a time.
What you can do to save money is buy one Mac Pro (maybe used) to use as a file server, then a refurb Mini for each employee, then create a network. Each employee will be able to access the server. You can use monitors other than Apple brand to save even more.
A MacPro is very nice for a Server, but is not (strictly-speaking) a requirement to run Mac OS X Server if you are cost-sensitive. Using a Mac Pro does allow you to easily set up a separate Boot Drive and a Mirrored RAID, which makes it a really good choice.
Mac OS X Server is not a compute problem, it is an I/O problem. So any Mac with good I/O power and multiple drives will do.
If you are especilly cost-sensitive, this could be a Mac Mini. One with a Fusion Drive is especially intriguing, as it provides most of the space of the larger spinning drive plus SSD as well as most of the speed of the SSD at an affordable price.
I run Time Machine as Backup for my Home Server, and it is a pretty good "hands-off" backup solutiuon for all the files on the Sever. But you do need to check it periodically to make sure it has not gotten stuck.
It is especailly important to Upgrade the Server-to-workstation connections to Gigabit Ethernet. Wireless is generally not fast enough. Otherwise, everything seems slow.