There should not be a problem if your flatsharer has the usual paperwork required to rent a flat (permit, job etc). You will get an attachment to the contract that the person sharing with you will sign and you will become co-renters.

I went through this procedure quite a few years ago and back then the only "problem" with the regie was that they asked us for a second caution. But - bad luck for them - my flatmate's father was a lawyer... and he told us that we didn't have to have a second caution because we were co-tenants having both signed the contract (so, in case of a problem, you have to solve this between the two of you). We wrote this to the regie and it was ok for them.... so they do know the laws, but sometimes they just try...