Heavy bag work I wouldn't worry so much about days off from. It is more cardio then anything and getting used to and practicing throwing. I think it can be good also to work on if done properly, as in say practicing moving from lower body shots to upper shots in combos. Lateral movement etc.

Now weight training, that is a whole different deal. It really depends on what and how you are lifting. But recovery is very important, plus what you are eating before and after...stretching etc.

In short...don't overdue the weights especially if starting out. The heavy bag...I wouldn't worry so much...just make sure you have your hands wrapped up correctly.