So you want to block off the right side ducting of the front AC. I'm assuming on the right side where it comes out of the AC unit. This would separate the two units instead of them sharing trying to cool the entire trailer. Will it work with half blocked off and not damage your AC? The answer is maybe.
You are going to have to get the make and model of both AC units. Look up their install manuals and there will be a spec of the minimum venting surface area. Then it gets fun. You need to calculate how much venting area is left by the remaining vents.
Not seeing your system my gut feeling is it will work just fine assuming 6 inch round vents. But if the system uses some tiny little vents then yes it might be restricted.