Hi Gregor
I use CAN-king to measure the CAN-bus load, generally I try to keep the load below 60% leaving some room for the servicetool communication.
If the load exceeds 60% or there about, I try to split the CAN-bus traffic or change some timing of the messages.
Expansion modules such as the OX and IX generate a good deal of traffic, however you can lower the frequency of which the modules send different feedback values.
So it all depends on how you set up your timing.
I believe your project could be done on one CAN-bus, but splitting it up in two would most likely save you a lot of trouble.
Anyway that's my advice.
regards Jakob