I do staggered hatches all the time, the few days of higher humidity won't hurt the other egg, as it's the average over the entire incubation period that matters...
Remember a bird sitting on eggs has little ability to change the humidity....
More so humidity isn't really what you should...
You can open the incubator any day, lock down is simply a suggestion that reminds you to remove or turn off turners and bump humidity levels it's not a hard fast rule that has to be observed...
As long as you can maintain the higher humidity levels for hatch (this isn't even a hard fast rule)...