MultiVersus

Saw a solution in the comments for projectiles but my timing wasn't working. Found this easier. It's based on time between knockouts as Crazy Dutchman has discovered.

Did this against a boosting partner/2nd Xbox in Custom Game Lobby. Local will not work.

Pick Superman.
Opponent Finn.

Settings
Game Mode = 1 vs 1
Knockouts = 4
Time = Doesn't matter

Map Rotation
Unselect all except Tree Fort.

When the Match starts move up to the opponent and hold to send them flying off the screen. When they respawn and land next to you hold again when they land and when fully charged it will release and knock them back off. Opponents get a spawn protection(dizzy stars) for a second and will dodge the attack if you hit them too early.

Video
https://gameclips.io/neocringer/224d53a8-8af5-47e1-9535-cae8...

Disclaimer: This is a 4 player method but works just as well if not way better than the solo methods and will save you a lot of time by getting all 4 of you all 100 doubles in less time than doing it alone and getting it for just yourself and takes less skill comparatively, the hardest part is simply getting the four people.

4 player method: (This is my double KO method as well) Find 3 other players either from Xbox LFG or by creating a TA session. Using this method you can get all 4 players over 100 takedowns and their 100 Double KOs in about 45 minutes to an hour depending on co-ordination.
Go to customs:
Mode: Free for All
Score to win: Highest scorer
Time: 15 minutes
Maps: Any that have a ledge already open on both left and right. I used Sky arena
Start by establishing the order in which you want to have people get the double kills, then have 2 people walk over the right or left side ledge to where if they get hit once they will fall off the side and die. Have person 1 use any attack to hit those 2 off the side and as long as they both die within about 1-2 seconds of one another it will count as a double kill. Now have two people go to the same side as before again, and second person gets two kills. Keep doing this until each of the 4 people get a turn. Now start with person 1 and repeat. If you can co-ordinate this well enough, each person should be getting 2-3 turns each minute making this a total breeze.