Perfect, that sorts all my doubts. Say neither iMac A and iMac B are not logged in and you log in remotely iMac A, what happens on the screen? Can anybody standing in front of the machine see what you're seeing while sharing?
I got mixed results. I do NOT know if the mixed results was because the client Mac was running Snow Leopard (10.6 - major release ago), but here is what happened.
When logged out of all users on my Mavericks (10.9) Macbook Pro, then started screen sharing from my Snow Leopard Mac to the Macbook Pro, it logged in as the primary user controlling the Macbook Pro screen and keyboard (anyone able to see my Macbook Pro would see what I was doing from remote).
So that no one would see what I was doing, I would need to first remotely login to the normal user account (maybe start the screen saver so that the screen/keyboard are locked from passer-by's), then start a new screen sharing session to the "Other" account, and now that account will start up in the background.
NOTE: If I logged into the "Other" account first, then used "Fast User Switching" to move the display away from Other to the normal account, then start a new screen sharing session to get back to "Other", when I tried to logout of Other, it put a dialog box on the Fast User Switched to Normal user's display asking if I really want to close all applciations.
That is to say, the normal message Other should have received, was instead delivered to the physical display which is currently displaying the Normal user's account.
Again, I do not know if this Fast User Switching behavior was because my client Mac was a 10.6, or if it is just the way it is. I do not have a newer client Mac I can connect to my Mavericks Macbook Pro to give that a try.
So I would say, if no one is logged into your iMac, you first login to the normal account, force the screen saver to start, then disconnect the screen sharing session, and start a new session to your Other account. That way you can login/logout without the normal users noticing.