can anyone clarify the difference

according to the OED, sardonic means "exhibiting bitterness, scorn or mockery" (mostly in the expression "sardonic smile"). It is not related to "saturnine", which means "sluggish, cold and gloomy in temperament". The only common ground of these words is their negative tone.

I can't totally agree with the previous, as in AHD4:

2a. Melancholy or sullen. b. Having or marked by a tendency to be bitter or sardonic: a saturnine expression on his face.

My take on this is that one would be actively sardondic but passively saturnine.

Yes, but that one need not be one and the same person. I mean, one could be sardonic without ever being saturnine.
And the other way around.