It's typical and recommended one tax professional handles the entire filing.

He asked for my help or advice in how to get onboarded as a vendor for professional property managers so I thought I'd ask here.

Many years ago, Home Depot and Lowes used to have professionals from each trade in those departments, most were retired etc, I find that less and less nowadays and find myself helping customers out when I hear an employee giving really bad advice, so just take advice in the big box stores with a grain of salt. 

I included in the proposal the benefits for the landlords, such as no worries for vacancies, a 5% profit share on top of the rental rate and the provision of a professional cleaning team for their property.

Readers are advised to seek professional advice.

Hey Gustavo, The double taxation issue will likely be experienced by someone in your situation without the proper tax professional involved.

Lesson learned Did you work with any real estate professionals (agents, lenders, etc.) that you'd recommend to others?

For example, looking at a property with "3" bedrooms but a giant loft area that could be converted to a 4th bedroom, and a full downstairs private area with a full bath that could have a convertible bed, allowing the property to sleep 10 minimum before adding any bunk or twin beds.

Lesson learned Did you work with any real estate professionals (agents, lenders, etc.) that you'd recommend to others?

You should really hire and talk to a CPA/tax professional.