I agree with Mark...inlet the upper tang first, then the trigger plate, which in effect, forms a lower tang. Your trigger position will have already been established and cut in the trigger plate, because you made a blue-print of the rifle and laid oout everything before cutting any wood - right?
I have never (yet) inlet a swivel breech action, but I've done lots of double guns, and I think the process is very similar. So, that would be the order of inletting for me at least.