OKay in my photography class last year this is what the instructor said to do to shoot fireworks, It was for film cameras, it should work for digital.
first off it is a must to have tripod, then if you have the option, put your manual shutter speed setting on bulb, & F28 or so, that way everything will be sharp, my film camera has a trigger release cord, so you don't have to touch the camera, so as not to shake it. I know my digital camera doesn't have this option, maybe you should take both cameras just in case. To have pictures w/ more than one firework in it, just take a piece of cardboard w/ you & between the fireworks put it over the lens & move the cardboard when another goes off thats in a different spot, since your cameras on bulb, it will keep capturing until you release the button, that where the release cord comes in handy, because I don't know about you, but I can't hold comepletely still for more than a minute. Hope this helps.
Tiffany