I learned: the simple solution at home with two PC’s is install x11vnc on my “headless” old second PC.
That works 
I tried in vain:
Tigervnc
Tightvnc
Turbovnc
Tigervnc-server
Commercial RealVNCDays later: tigervnc-server and Turbovnc … works if you are aware of:
very important root tip !!!Learned that the hard way
For clipboard synchronization i use autocutsel and to connect to x11vnc i use vinagre .
As startscript in XFCE ~/.config/autostart for x11vnc i use:
/usr/bin/x11vnc -display :0 -forever -bg -noxdamage -o ~/x11vnc.log
Call autocutsel also from a script in ~/.config/autostart
Hope thats usefull for someone else !