To walk from the United "normal" gates to international terminal is about 10 minutes. By then you are at check-in. Security at intl is never more than 10 minutes, 15 at most if there is a rush. If you could print your BP in advance you wouldn't even need to leave secured area.
Unless OP leaves the airport, there is no need to re-clear security at SFO. There is an air-side walkway between the United domestic gates and international terminal G.
There is also a chance that the MCO flight arrives at T1 rather than the usual T3. This is a recent change at United at SFO. If this occurs, you can still connect air-side via a shuttle, but I would leave extra time for the shuttle.
Given the state of affairs at United -- namely, frequent delays -- I would NOT schedule a two-hour connection on separate itineraries. If they are a single itinerary, meaning you will be protected in the event of a misconnect, that is fine. If you want to visit the City, then by all means, stay 12 hours.