There is massive debate surrounding this topic. Many say that cardio is counterproductive which can be very true, and many say that cardio can help your body and your muscle development. Many competing athletes are forced to do cardio to stay as lean as possible while maintaining muscle. So I guess the real answer is that there can be pros and cons, but cardio done correctly can help you overall.
If we overdo the cardio, we will be burning up too many calories and the idea with building muscle is taking in surplus of calories. So we need to balance it that we are not burning up too many calories. Cardio should also not be done for too long. Maximum 20 minutes cardio I would say at a steady heartrate of 120-130 bpm. Slowly walking on an incline on the treadmill or doing the stepper for 20 minutes can be perfect. Furthermore this can improve our flexibility and our overall recovery time. And we need not forget the most important factor, cardio strengthens the heart. Many heavy weight trainers neglect their cardio health and also with the widespread use of steroids heart health is important. It is wonderful that we can deadlift 220kg but can’t walk up a flight of stairs without having to stop.
High intensity short bursts can be a brilliant method of cardio and also aid in building muscle. Much like boxers will do when training. So short sprints are brilliant for the legs to build muscle making sure you have sufficient rest periods. Sprinting on an uphill on the treadmill for 20 seconds then 1 minute break is excellent as your heart learns to lower the bpm faster. This can strengthen our muscles and our heart.
So all in all it can be very important even if only done twice a week. Find some time to walk on the treadmill or lightly cycle or even attempt that stepper. In the long run it gas great fat burning benefits as well as cardio strengthening properties. Simply make sure you are taking in extra protein and make sure you are not burning too many calories.