This is my own opinion, shared by most paper conservators, mainstream art curators etc., but not necessarily by dealers and collectors, is of the "less is more" school of thought.
According to that line of reasoning keeping something in as close to its original condition as possible is the first consideration. Especially if that original condition is rare. A sheet of stamps as opposed to individual stamps. Toy in unopened box vs opened and damaged. Unjoined poster parts vs joined and pasted to a single backing.
If the poster paper is in good condition and framing the parts separately makes any kind of visual sense, that is a good solution.
Second choice would be to join the pieces using pressure sensitive "tape" made from Lascaux HV360 and Japanese paper. Hinge with the Lascaux/Japanese paper or wheat starch/Japanese paper to rag/alphacellulose matboard faced rigid backboard (aluminum honeycomb panels joined to strainer would work - you could consult with SmallCorp re particulars of construction). And create a "window mat" from aluminum or sealed wood fillet lined with rag/alpha cellulose matboard using appropriate stable adhesive to cover edges of the poster and hold in place. UV filtering plexi, don't know how thick, maybe 1/4"?
This would alter the original only minimally, and be very reversible. Display lighting and control of that lighting should also be a major consideration as I'm sure many of the inks are colored with dyes that would fade quickly.
Sounds like a fun project, but would involve more than just framing - the environment it will go into should also be carefully considered. Having a paper conservator on board, even if only as an intital consultant, would help the owner plan for the long term care of the poster.
Best,
Rebecca