At the current state, this achievement is kinda buggy. For 4 months between December, 2017 and March, 2018 the server just simply didn't register our Weekly Challenge times, no matter how many times you completed both of them. I suggest you to start doing this as soon as you start playing the game!!!
All you have to do is to compete BOTH Weekly Events in 2 weeks in a row. The servers are refreshing on every monday, 12:00 (Central Europe Standard Time) so you can check which tier you finished and you can go for the next Weekly Challenges. On the third week, you should get the achievement. If for some reason it doesn't pop, it mean that one of your times wan't registered, so just keep trying and trying until you get the chivo. It is a pain in the ass, but there is no better solution for this, until Codemasters decide to fix the servers or the communication problem.
Edit: Several people in the comment section says that you only have to complete one weekly event for the achievement. It might be true, unfortunately i cannot confirm it since i already unlocked the achievement. If you really not sure about it i still suggest you to complete both of them. Not just to be a 100% about the completion, but you also earn more credits what you can use to improve your team and garage furthermore.