Here's a quick and easy tip to combined multiple fireworks in one frame. For my photos, I've used a tripod and shot each firework explosion with roughly a 2 seconds exposure (shutter speed). Then with all the photos, upload them in Photoshop as layers. Below is a summary of steps from the video.
1.) Starting from the top layer, change each layer to "Screen" blend mode 2.) Add the Curves Adjustment (Window > Adjustments > Curves) 3.) In the Curves Adjustment drag the middle line towards the bottom
Normally to capture more than one fireworks in one frame, the camera should be set to "Bulb Mode" with 30 sec. or longer exposure. With such a long exposure, any camera movement would blur whatever background/foreground you may have. Instead, I've took a bunch of capture of the fireworks with a 2 sec. shutter speed. Combine a few photos in Photoshop to get that dramatic firework show in one frame.