I've been training multiple styles for the last 15 years, but only the last 8 seriously.
I used Martial Arts as a cardio workout, and I make sure that if after my martial arts workout, I didn't burn enouph calories, I'll just start kicking and punching until I'm spent. But I also do makiwara and power drills 1 or 2 days a week in which I don't lift the same muscles or do cardio.