If a skill is much higher than a related skill, the higher skill would take longer to pop. The opposite goes for the lower skill. In terms of difficulty popping several weeks in a row, the lower skill would have a decent chance to pop 2-3 weeks in a row (in my opinion).
You don't have to train the same skill 2 weeks in a row. Whether or not you should train the same skill for consecutive weeks factors on several things. Did a skill you wanted a pop in not pop the previous week and you want to try again? What is the level of said skill in comparison to the other skills of the player?