[McAroni]

I would try to combine all the pdf you need in one song and arrange them in the order you need. for example:
1. double bass
2. score
3. double bass
4. piano part
5. score
in each pdf you define the pages you need.
this should give you a song with the correct order of the sides.
See also:
https://youtu.be/FOZ5CO00nRk
please note: if you need the score twice, for example, you have to add it to the song twice.

as a wish for @mike: would it be possible to duplicate the files used in a song directly in the edit files dialog of the song without the add dialog?