flash guide number

I have just purchased an Olympus 8080 digital camera. I do alot of picture taking at my church and school, in large auditoriums and meeting rooms, and I am tired of getting dark backgrounds when just using the built-in flash. I therefore am going to be using both the internal flash along with the Olympus FL-20 external flash (FL twenty)

If operating in manual mode, how does one figure the flash guide number when using both built-in and external flash?

I think you should probably ignore the built-in flash. Its guide number isn't going to change so I don't think it's going to have any effect on your exposure. It's only the more powerful flash that you need to consider.