Do you have a monopod? If so, use that. It'll give you a much more stable platform than handheld, and you should be able to use it even in those cramped little seats most auditoriums have... I used one at our son's graduation (college) last summer, and it worked out pretty well with the Promaster 70-300 on K100D rig I used. You'll still have lousy lighting to deal with, but your lens will be a lot better off with the pod than without.
Another point to remember is which direction the grads will be walking across the stage, so you can position yourself to catch their faces. Try to get seats where you'll have a clear view of the stage, without tons of people walking in front of your camera
...
Take some trial shots before the students arrive, so you can get the settings as good as they get. Set your ISO high as you feel quality is OK. Set the WB for the lighting inside (something I wish I had done...). Hand the other camera to your hubby and have him get shots too. We had both my K100D and my wife's little Nikon P&S along. Her shots actually came out better than mine, for the wider-angle pics of the audience and student hordes. Mine were much better for the zoomed in shots of our son and his friends and the actual stage-crossing. Use the strong points of each camera.
And, of course, remember to have fun
The pics *you* take are the ones for memories, the ones from the official photogs are the ones for hanging on the wall (at least that was our philosophy).
Jim