Adequate rest times should indeed be taken into consideration.

After a 10RM set, it is likely that available ATP and PCr sources are utilized. Normally at least 2 minutes rest is required for recovery between efforts. Additionally nutrition will impact significantly on the ability to perform second sets. Without muscle glycogen stores at high levels each subsequent set is likely to be more difficult. So stick with the 2nd set at 9 reps. Add weight when both sets can be completed for 12 reps. Objective markers are necessary,