I have a 6 x 12 enclosed I staggered the front bike two feet off of the front wall to the right. The second bike is centered just past the trailer axle towards the rear to the left of the trailer to keep the tongue weight manageable. I would recommend putting the front bike in first where you are comfortable and then bring in the second bike a watch you trailer tongue and trailer axle load. Move bikes to get a almost balanced load with still enough tongue weight to keep the trailer from whipping. I know of no exact science for this except feel. I also placed a bracket for my removable wheel chocks centered in the trailer with still enough tongue weight to keep the trailer from whipping for when I haul only one bike.

 

7x12 here too. I set mine up with the first chock and centered at 19 inches from wall. Other is 2 feet to the rear of this centered about 16 inches from the wall. This worked great for a Road King and wifes Sportster. Only problem, and something that I did not plan too well is that the RK's bags were really close to the highway pegs on the Sportster so I felt better removing the bag while traveling. I am looking for a total width measurement on the 09 Ultra to see if I have to add another chock mount plate or move the existing. The total width (bars) of the RK was 33", so there was 5 " between the bar ends and side of the trailer. Anyone know the width of the ultra?

 

I guess I forgot to add that. The bars on my bike are 42" from tip to tip. I havent measured my buddies bars yet but they are much narrower than mine.