How to remote access a server while using a client user ?

If you mean by server the H/W, then If for example you are using Dell servers, there is a port for managing the server called integrated Dell Remote Access Controller (iDRAC), just configure the TCP/IP from the LCD panel then you can access the server remotely from any client computer so you can manage the server.

If you mean by server the OS windows server for example, then you can manage it through remote desktop connection by typing in the run (mstsc) which means microsoft terminal service connection and use the credentials to access the server remotely. 

Normally we can access the server , with few steps

they are 

 go to Start - > run -> mstsc command 

then we get a console page or application for remote support , in that give the hostname name and server ip address with its proper subnetmask. then click connect. 

then enter your respective login username and password.

then you can take remote support from the client machine ,,, its simple