I also have two cubes and that is also my plan. Yesterday I flashed one as a CUN with this guide: HOWTO: Reflash MAX!Cube to CUL/CUNx and use with homegar and homematic binding
Homegear talks to the cube via the network and not via USB.
I installed homegear with APT (I did not know I could do it with openhabian) and connected all devices yesterday evening. The process was very fast and without any issues. The OH homematic binding was able to find all devices and now I am playing around with homegear. HG also has a nice graphical UI for setting the heating schedules and can also direct link e.g. window sensors and thermostats like in MAX.
Main reason why I did it was because yesterday I tried to move a device in the MAX Software to another room and it did not work and the only solution was a factory reset as usual. That was the final nail in the coffin. 
I may exchange the other cube for a Homematic IP LAN Gateway and one thermostat just to play around with it.