Yes, synergists can combine their degrees.
But, in your example, Kanon, an Imber synergist, wouldn't be able to summon fog. Fog would be in the domain of the Pulvius Degree.
However, an example of a "mixed" degrees would be the creation of lava (Accendo and Pulvius). However, combining degrees can be tricky. I'll try to have a General Degree Information post put up some time soon. For now, feel free to ask me any and all questions you have. (: