Windows Server permits two simultaneous remote connections out of the box as described in this MSKB document:
At any point in time, there can be two active remote administration sessions [to the server]. To start a remote administration session, you must be a member of the Administrators group on the server to which you are connecting.
No special configuration is required other than to enable Remote Desktop in System Properties and logon with a user account that is a member of the Administrators group. Further, each of you should either:
- Logon with separate Windows user accounts, or
- To use the same Windows user account but in separate logon instances, edit the local group policy setting
Computer Configuration/Policies/Administrative Templates/Windows Components/Remote Desktop Services/Remote Desktop Session Host/Connections/Restrict Remote Desktop Services users to a single Remote Desktop Services session and set it to Disabled.