On that size trailer, the best setup is to place three wheel chocks, with the center one being a foot or so further foward than either of the side chocks. That allows you the option of carrying a third bike and, trust me, the situation always arises when you need to carry more bikes.
With that setup, you can put the chocks about 18 inches apart.
If you go with the two chock setup, measure from the center line of the bike to the end of the handlebar, double that number and then add the number of inches that you want to have between the handlebar ends of the bikes (4-6 inches should work). BTW, the measurement and doubling is the same as the width of the handlebars, so you can do it that way.