Remote desktop / screen sharing / VPN questions

Question

Hi all-
OK, I am trying to set up our all-mac environment to do something in particular - but I am only 'novice' level at VPN stuff and 'noob' to the Apple Remote Desktop and Screen Sharing stuff. I already have screen sharing set up, and also have back to my mac set up - but these both seem to be 'whole computer' control methods, where the remote user takes over the entire target machine. I have a different need, but don't know what terms to use to describe it or how I might achieve it.
Environment overview:
2014 iMac, 2009 13" MacBook Pro, 2009 17" MacBook Pro - all running Yosemite.
2013 AirPort Extreme handling the LAN, both wired and wireless. iMac is wired, MacBooks are wireless.
iMac: Two users (UserA and UserB, both admins).
MPB13: Two users (UserB as admin and UserC as standard).
MBP17: Two users (UserA as admin and UserD as standard).
I am wondering if this is possible to do, and if so - HOW? :
UserB sits at the iMac, and is using it. UserA sits at MBP17 and wants to get something done on the iMac, so UserA logs into MBP17 as UserA then remotes into the iMac as UserA and does what they need to do in UserA's iMac account - without making UserB (who is still sitting at the iMac and using it) stop what they are doing.
Hope this makes sense, and am looking forward to your input.

Answer 1

UserA must have a user account set up on the iMac that is different from UserB. With this set up when UserA initiates a Screen Sharing session with the iMac from the MBP17 they will just need to log in with their user account. When they do that a dialog box will pop up asking if they want to share the display with the user currently using the iMac, in this case UserB, or if they want to connect to a Virtual Display, which would give them their own environment separate from UserB who is already logged in and using the iMac.
Here is an example of the dialog box UserA should see when they attempt to start a screen sharing session with another Mac in which another person is already login in and using the Mac. In this case UserA will select the option to connect to the Virtual Display which allows them to work on the iMac without disturbing the user that is already using the computer. Note that the user already using the iMac may experience a slight performance hit because two users are using the resources on the one computer.
As for the acronym VPN, it stands for Virtual Private Network which is a protocol for making a encrypted secure connection over a public network between two or more separate networks. Screen Sharing actually uses a protocol called VNC which stands for Virtual Network Computing.