With friendly help by Frederik I finally could resolve the issue, and it’s as easy as I expected:
In my trytond.conf, I had the line
listen = localhost:8010
but it was not under the [web] statement.
So here is the solution for simple-minded people like me. To operate two local Tryton instances on one machine, with two GTK clients at a time, you need to
create an entry in trytond.conf, looking like this:
[database]
all entries needed here
...
...
[web]
listen = localhost:your-port-number
at GTK client, at the “host” box type in:
localhost:your-port-number
your-port-number must be <> the default value 8000.
All of this is very obvious to you nerds. It is not to us users.
Thanks for all your effort,
Wolf