Maarkr
there's a push-pin looking thing in the upper right corner of the open plugin... enable it and it will keep open when the next plug is opened.

 
Thanks again for this helpful tip!

While on the subject of this, I have also noticed that plug-in windows need to be ctrl + dragged to the multidock if you wish to dock an open window.
 

Pragi
Thanks, Wookie,
I tried your hind and every other constellation possible under vst preferences-
Recycle Plugins Window,
but there´s still only gui at a time opening.
 
Any other idea?
 

Confused, as before I replied I tested it and it worked as expected i.e. I inserted two plugs and both GUI's are visible.  I will do another test and see what happens.

OK just tested it and it works as expected i.e.
 
With the Recycle Plugins Window ticked only one GUI shown
 
With the Recycle Plugins Window un-ticked two GUI's shown

Though I do note the the second GUI overlays the first GUI and consequently if the first is smaller than the second it can appear hidden.
 

Yup, I noticed as well and just thought it was new behavior... kinda liked it after awhile because I have made changes to the wrong track more than once when I have had multiple instances of the same plug on the screen from different tracks.
 
Good to new I can turn this on and off though.
 
Thanks guys,
Jim