Joint business with two people - how to do if both have URSSAF?

I am already registered as auto-entrepreneur profession liberale business in France. I pay my taxes with (Unions de Recouvrement des Cotisations de Sécurité Sociale et d’Allocations Familiales (URSSAF) with the Impot deduction included. I have started a project with a collaborator, who will also soon be registering for the same status as me with URSSAF. Our interests, and costs for the business (domain name,  publicity and all expenses) have been equally shared and will continue to be.

1. He is currently a student and we have heard that he could apply to URSSAF in a special way and benefit from a reduced rate on tax if he works for himself / creates a business on this scheme. I currently pay 25.3% but we heard that the reduced rate is available to students or younger business creators and might be as low as 5%. Is this true?
2. The joint business would not be my only income into my URSSAF declarations - as I do other work that does not involve him. If he can go on a scheme for reduced tax, is there a legal way that our joint project can benefit from his low URSSAF deductions, without me then having to pay again (an additional 25.3 % on top) if we already paid 5%? As the project’s income would have been put through his URSSAF declarations already.

To create a separate company just seems too expensive and neither of us has the money to do it right now.

Maybe I should clarify; I said auto-entrepreneur but also profession liberale, because my URSSAF déclaration de recettes form includes a 0.02% deduction for formation professionnelle liberale obligatoire. Actually I am trying to set up a bank account for myself too, and find the distinction hard, as some banks offer accounts for something like Pro for a business account, or instead make a difference between auto-entrepreneur or profession liberale. This gets confusing for me.