I was asked to by a homeowner to help with a wiring project. She has 1 200 amp main panel and 1 100 amp subpanel. Both are full yet she needs more circuits for a bed n' breakfast. Can another 100 amp subpanel be installed?

Yea, but I think it's time to do a load calculation and determine if a service is required. Say go from 200 A to 400 A.

If the utility offers it, it can be done, such that there are two disconnects outside that feed two 200 amp panels.

What happens when everyone uses their hair dryer at once and there is cooking, heating/cooling and hot water heating going on?

If it's a few circuits and the panel is full, there are ways of adding separate circuits in the full panel as long as there are "poles" available. The number of poles the panel supports should be on the panel. Count the wires going to the breakers that will be the current number of "poles". The difference is the number of wires that can be added.