A few things of note; The 'larger' forcefield will always take damage first, meaning that whichever force field has more hitpoints will take damage (because it is larger)
However, for a few things.. Once upon a time, forcefields had armor (i think this was removed, unfortunately... i really liked the concept of a low hp ff with high armor being harder to kill but vulnerable to siege weapons). In any case, If a shot will fully kill a forcefield module, it will just die, and no overkill damage is dealt to the riot starship. This means even a mk3 siege starship will take a full shot to destroy the forcefield, and an additional full shot to destroy the next forcefield, despite it having major amounts of overkill. This makes riot starships considerably awesome at tanking.
A final word - I tend to lose the most riot starships when i run out of resources. If my riot starships cant be fabricating replacement shields, they are nowhere near as tanky.