Plug in one wireless (DECT) "master" base station that can serve two wireless handsets. You now have lost the party line/conference call function.
That seems strange - are you 100% sure of that?
I have a DECT base station plugged in upstairs and the main phone plugged in downstairs. Both share the same line - and my broadband modem and Sky receiver are also on this.
I can talk and listen on both phones simultaneously, exactly as if they were both wired phones on the same line.