Is it possible for two users to remotely log in to a Mac simultaneously?

Solution 1:

If what you need is to connect to the same Mac machine with two different users in two different sessions, as Terminal Services for Windows can do, this is the answer: currently, with the default system, this is not possible. However, alongside the future Mac OS X 10.7 Lion —that will be released summer 2011— it will be possible to achieve with the updated Screen Sharing.

If what you need is to connect to the same Mac machine with the same user in the same session, you already can connect to that machine with any VNC client, if Screen Sharing is enabled.

Solution 2:

Yes - Lion makes the distinction between sharing the current screen and letting a different user log in. This brings multiple login to Mac OS X in an official way.

When you connect from another Lion mac - this window shows how you can select either access method.

A few VNC client apps (Like Screens for iOS and Screens for Mac supported this when Lion was released. Several popular VNC clients didn't work initially due to this change (and may need updates to work with Lion's new screen sharing method).