Hello @Bmuir,
Creating multiple sessions for the same user account is supported in ThinLinc, but generally not by modern distributions and graphical desktop environments. Hence, this is not a limitation of ThinLinc, but more of how the whole ecosystem with graphical environments in Linux distributions has evolved.
See Why does starting multiple ThinLinc sessions as the same user sometimes fail? for more information.
Happy to hear that you got it resolved, though! 
Kind regards,
Martin