You only need two Cores if you’re going to be engaging in activities where you want to see the activity from your upper and lower body at the same time. For example, if you’re a rower and you want to see the muscle activity for your legs and arms at the same time, you would need two Cores.
If you typically work out your upper body one day, and then your lower body another day, or if you are okay with just measuring one part of the body (either lower or upper) at a time, you will be just fine with a single Core. You can just swap your Core from the top to bottom as desired.
For example, when you’re doing squats, you can put your Core in your Shorts or Capris, and when you do bicep curls you can move it to your Shirt and then back to your bottoms when you’re doing lunges.