flyerCO is correct, you won't be able to upgrade a companion in advance if they aren't on the exact same itinerary.

When you arrive at the airport, you could go to a WorldClub if you are a member; if not, you might consider going outside of security and tell a check-in counter agent (or WorldClub rep) that you would like to try to upgrade a companion, and ask if they could put his name on the list. Nothing will be processed until aprox 30 minutes before the flight departs and at the gate, so it may not be worth the bother. Since companion upgrades are near the bottom of the upgrade hierarchy, you might just inquire once a GA shows up.

You're only able to upgrade one passenger, and flight attendants have repeatedly told me that they can't make seat changes once on board. In the past I've only been able to use Plat. companion upgrades about 50% of the time, as elites themselves have filled it up. Since MCO is largely a tourist destination, there's probably a goodr chance there will be fewer elite flyers vying for first. Good luck.