My question involves landlord-tenant law in the State of: California
I signed a month-to-month lease with another girl 3 years ago. We were both on the lease. She gave a 30-day notice and left 2 months ago. The new property manager said I'd be the only responsible to pay full rent until I found another roommate. Since then I've been paying full rent. Now I found another roommate and the property manager gave us another contract. The problem is that both, my new roommate and I are in the contract, as co-tenants, but we're supposed to sign an addendum that states that "I understand that I'm responsible to pay the entire rental amount each month on the 1st, and that it will be my responsibility to collect my roommate's portion of rent each month..."
Why do I have to sign that? If we're co-tenants we are supposed to have the same rights and the same responsibilities... I feel like if I sign that, we have the same rights but I'm the only one responsible. I told the property manager I didn't agree and explained why and she said "the addendum has to be signed or the lease is not valid. No exceptions. You're the sole renter right now and have chosen to have a roommate. These terms are required for us to add your roommate."
The new contract says that we're both co-tenants not that I'm the sole renter. I'm the sole renter now, in the lease I have now (well, the one I had with my roommate but she left already). We want to be both in the lease and both be responsible.
How does that work?
Thanks for any advice you can give me!

