So this is a question I get asked very frequently, and also when pairing two muscle groups, how many exercises should you perform on each to ensure this session isn't 2 hours or longer!
So my preferences and recommendation for pairing muscle groups would be, choosing a main muscle group such as Back / Quads / Glutes / Shoulders, and adding a smaller muscle group which requires less exercises, such as your biceps or triceps, your calves, and perhaps hamstrings unless you wish to really improve this area. In that case I would put that as a main focus in your sessions with your second muscle group being one which doesn't require as much focus as your hamstrings for example.
So a very common pair would be, Back and Biceps, Shoulders and Triceps, Glutes and Hamstrings, Quads and calves. That's a great split to work your overall body, however as discussed above, if you have a weak body part of one you wish to focus on more this training split will need amending to suit you.
I personally do not need too much upper body development as it is a stronger area for me in terms of the class I compete in, which is bikini. Therefore my training is 2 upper body days in total, and 4 lower body days. These are divided into Quad/Hamstring/Glute and Calve training.
You don't want to pair the wrong muscle groups together, if your really wanting to improve two body parts keep them away from each other as you will become fatigued after you've focused on your first muscle group before hitting your second.
How long should you spend on each muscle group when pairing muscle groups together?
So of course, we don't want to be spending 2+ hours in the gym (a typical gym weights session should last around 60 - 90 minutes - now it can last longer or shorter, it's quality over quantity in my opinion).
So if you're focusing on for example Back and Biceps, you don't need to perform the same amount of exercises on biceps as you do back, for the simple reason biceps are such a small muscle group, and as females we do not need to overload our biceps with volume and sets/reps, unless of course you WANT too. If you're really wanting to improve your arm training I would recommend having an 'arm' day which will consist of biceps and triceps, but this isn't something I would personally do as I feel that would be counterproductive towards my goals.
As I was saying, I would recommend 5-6 back exercises, varying your rep ranges and sets depending on what exercises, and depending your goals, and around 2-4 exercises for biceps.
The same would apply to the majority of paired muscle group workouts, performing the majority of your exercise on the first muscle group, and 3-4 on the secondary muscle group.
You can have your own tailored training plan designed for you from Fierce & Fit, click the link below to enquire about prices and information.