Can VNC run a stealth, remote desktop control with concurrent session?

As the title says, I am looking for a solution, free or paid (with reasonable price), that allows me to control a remote desktop (PC) with these criteria:

  1. Stealth/hidden: the end-user (PC) must not know that the PC is being controlled remotely
  2. Concurrent session: the remote desktop control is possible even when someone is using the PC

I believe (1) is possible with Ultra VNC, but I’m not sure about (2). Is it possible? or could you suggest a software that is able to do this?