Hello Lilian,
you do not need any middleware but this can be tricky. You need to configure the web server (nginx? apache?) to map some URLs into mezzanine and some into web2py.
The problem is if they login into mezzanine and then go to a web2py page, how are user credential transferred? I can think of many ways to do this but all require coding and some form of data exchange between Django and Mezzanine (JWT? Cookies? Shared database?). If Mezzanine support Cas or OAuth, then you can have web2py plug into those.