Get the FP for RSR first. That FP is actually located way before you get to Carsland. They moved it right next to Bugs attraction. They go very fast. Follow the people rushing to get those FPs 'cause that's what many people do when DCA opens.
You can only have one FP at a time. You can't get another one until the time on your current one passes. So, if your FP at RSR says 4-5pm, you cannot get one until after 4pm. The FP machines know what you have based on you inserting your ticket. The machines will simply not print one if you still have an active FP.