Don't sweat having multiple concurrent reservations - Marriott and its properties will gladly take reservations for as many hotels as you and your friends can check in and pay for.

Just reinforcing SkiAdcock's reply, just make sure at least one of your friends is added to the room reservation for Milwaukee. If these are one night stays, you are likely to get credit for both stays. It's when you've got multiple rooms across multiple hotels in multiple cities has Marriott in the past gotten back to me to find out where I was for real and what was going on (I'm in one city with one of my kid's teams, booked.room for my wife in another and then, after having checked in the team, had to drive to another city and stay at another hotel for one night in order to support an early morning meeting, etc.)

David