Hello,
is it possible to run two ovpn servers on one RB?
Via Winbox it seems not to work.
Thanks.
No, only one OVPN instance can be running on one router.
Why this? Will this be changed in one of the next releases?
What is the difference between the button “OVPN Server” in PPP and the adding of a new interface “OVPN Server Binding”?
So it would be possible to create more OVPN server bindings configs but only one server instance. This makes no sense. 
Please read here what binding is used for:
https://wiki.mikrotik.com/wiki/Manual:Interface/OVPN#OVPN_Server
Thanks