For the first one you put a white matte (or desired image/colour) on the lowest video track and any video clips you want to apear on screen at the same time on separate tracks above. Then adjust the scale and position properties to each to get your layout. You can also crop the clips.
For the second you need to set the camera on a tripod, shoot the singer (in this case) each time without making any adjustment to the lighting or camera. Then as before, put each video on separate tracks (no need for the matte on the lowest track) and crop the sides of each clip until each of the singers apears on screen and the backgrond matches up.
You need to take some thought on positioning while shooting.
Also, remember to shoot everything with your camera settings on manual (focus, exposure, white balance, etc)... if you shoot with your camera on automatic, your camera may adjust different settings, resulting in your clips not matching fully.