This achievement is very straightforward if you play in only one system. Just log 14 days in a row, making sure you get the daily login challenge everytime. It should pop in the 14 day.
However, if you play in multiple systems like me, it can be a pain in the ass.
In the end, what helped me was to always use the same machine (win8 or x1) to get the daily challenge. Do not mix, as it will reset your streak. Whenever you unlock this achievement, just wait for one more day, log first on the other system, and win both achievements.
UPDATE: As per QuickMythril's comment, it doesn't seem that you get both achievements on different systems with the same streak. So i recommend going first for one of the platforms, and then another 14-day streak for the other.