A player of mine had an interesting concept where two parties who are opposed to each other would play at the same time in the same session. In essence the plot would be a group of thieves have been hired to steal some kind of magical artefact from a castle, and the first part of the session would be this group infiltrating the castle and getting to the artefact.
Once they remove it however, of course alarms will go off, or a pair of patrolling guards will see it's missing on their next rotation. This would then introduce the second party who are a group of elite guards.
I was thinking the easiest way to run it would be to give each group 30 seconds to decide their course of action, and then act upon it. The guards will have a map so they can point out to the DM where they want to move next, and the thieves will have a map revealed to them as they travel along that they can point at to try and avoid metagaming as well as other punishments for metagaming. Naturally the two parties will be at opposite ends of the room.
How can I run a session like this where there are two opposing parties in the same session?