Well there are pro's and con's to doing cardio and resistance training together.
The major advantage is only doing the one session per day...so that's a convenience factor. Another advantage is, if you limit the total workout duration, the amount of cortisol (a stress hormone) produced can be limited. Cortisol inhibits protein synthesis, or, the muscle building process. I have read that performing 2 high intensity workouts a day can increase cortisol production by 4x, relative to a single high intensity workout.
Disadvantage wise, upon the completion of your resistance workout, if you choose to perform cardio, you will be "starving" your muscles for a longer period of time. For example, if you perform heavy sets on back day and then spend 15 minutes performing HIIT cardio, that's an additional 15 minutes of increased catabolism (muscle breakdown) in your back muscles.
The other major problem with combining the two forms of training is the different stimuli. If you are training for muscle mass, then your weight training would be relatively heavy and thus stimulate protein synthesis. Conversely, HIIT cardio is primed at improving muscular lactate threshold, VO2 max, anaerobic fitness, fat metabolism and so on. The two don't work optimally together. It's like telling someone to multi-task: you cannot focus 100% on either task and therefore not receive 100% results.
In addition to all of the above, combining cardio (for fat/fitness) and resistance training (for muscle) will have a much more significant negative impact on males than females due to hormonal differences between the sexes.
Weighing up everything, ideally, you would want to separate your cardio and resistance training by 8 hours. Many athletes train this way. However you do need to take other factors into consideration, such as your lifestyle, energy levels, level of fitness etc.
