Hi,
this cannot be done. Only one GUI version allowed at the same time on you client.
However, you could install DP 8.1 GUI on your client and connect to DP7 cell server with minor messages. I recommend to use the new DP GUI on your client and connect to cell server using RDP; if not sufficient, you may try "psexec.exe \\servername -u domain\user -h cmd.exe" followed by manager.exe.
Best regards
Daniel