Aggressive territorial cat to another cat

TDLR: cat bullying another cat

I have a 5y/o female cat (M) and recently a new roommate moved in with an 8y/o female cat. M is curious to be friends with M and has lived with cats before. T hasn't lived with other cats. Both are spayed.

T is often hissing at humans and running away. It's been 3 months of M and T living together and T has become increasingly territorial (blocking M from my bedroom/her safe space, coming into my room to eat her food and use her litter box, swatting and hissing at her). M has been quite stressed so she started peeing in my bed to avoid using the litter box in the living room. I temporarily moved the litter box in my room so it's easier access for her.

We didnt have much of a slow intro other than T was in my roommate's room for one or two weeks before T was allowed to wander the home.

My roommate will be living here for another 8 months at least so looking for any ideas to reduce the aggression and handle this situation. We're located in Canada and both have access to vets.