How to display 2 part movie?

That’s not possible.

You should put one of the versions into a separate folder and mark it as a separate {edition}.
https://support.plex.tv/articles/multiple-editions/

Another approach to consider is to merge the two files into one.

If this produces a perfectly playable file, you may be better off by generating an optimized version of it in Plex, instead of maintaining the iPlayer version separately.
(unless of course the iPlayer version has something that the BR rip is lacking)