Hello Nick,
I would try to fit the nikon 200-400 with your bodies in the lowe pro and
leave the nikon bag at home. I have the nikon 200-400 my self and never use
this bag because it has in my opinion not the protection a camera bag offers.
If the other lenses do not fit any more in your bag I would try the Think tank skin body bag
or something similar which can hold two smaller lenses, you only need to buy an
extra "koppelriem" for this bag. I use the skin body bag in combination with a think tank airport
acceleration and this has worked pretty well for me. One advantage with the Airport Acceleration is that it especially designed for airplane travel and it is les conspicuous in regards to a normal camera backpack.
And when traveling with my camera I only put the most important items in my camera bag
such as a charger, memory card, image tank, wereldstekker and my camera cleaning kit all other stuf
will be in my hold baggage.
I hope this helps you a bit more in making your decision.
Regards,
Ronald Noeken