ATTENTION: Many have reported this Acheivement as glitched, including me! Here's how to get through it.
First, load the Exhibition Game, Snow Day game mode (if you set it at 2v2, you can also unlock the Double Up Acheivement) with the 1 Goal game mutator. When selecting a team (Blue or Orange), leave it on Auto select, as many have found that to cause them not to get the pop. Play however many games it takes for you to win on both teams this way. However, if that did not get you the Acheivement, like me, then this is what to do:
1) First, make sure this is your Home Xbox (go to Settings > Personalization > My home Xbox).
2) Then, kill the Rocket League app/game.
3) Then, go to Settings > Network
4) Click Disconnect Wireless
5) Click Go Offline
6) Go to My Games & Apps, then Manage Rocket League and delete your Profile's save file (not the reserve data file).
7) Then launch Rocket League. It will launch you with a fresh save file.
8) While offline, with your fresh save file, repeat the above solution.
9) Then, kill the Rocket League app/game
10) Go to Settings > Network
11) Click Go Online
12) Set up your Wifi network again.
13) Then launch Rocket League. It will sync your legitimate, older save from the cloud.
14) The achievement will then pop.
(Credit to OdadjianActual for fixing this bug in the Mad Scientist Acheivement)