After the joined statement that was published on Milan's official website, Inter and Milan could both play in a San Siro totally owned by the two clubs (acquiring it from the city of Milan) as it's their first option, even if other ones could be explored too. Here's the full statement:
"A.C. Milan S.p.A. and F.C. Internazionale Milano S.p.A. announced the signing of a Memorandum of Understanding formalising their willingness to work together on the project for a modern, state-of-the-art stadium.
An infrastructure adapted to current standards is paramount to delivering a world-class fan experience, as well as on-and-off-the pitch success. The Clubs believe that a shared stadium will serve in the best interests of all stakeholders from a financial, administrative and technical perspective, and are evaluating several possible options, including a complete renovation of the existing San Siro Stadium. A joint task-force to analyse all options accordingly has also been re-established.
The Clubs aim to conclude an initial evaluation phase by the end of the year, in order to commence the technical and administrative activities required to deliver a modern stadium to their fans in the shortest possible timeframe.
Today the two Clubs met with the Mayor of Milano re-opening a constructive and productive dialogue with the objective of examining in more detail the option of a redevelopment of the area around San Siro".