Here is an overview of some ball sport mods that use 2 and 4 balls, respectively.
I started from oakyball.tbm by Oaky, except I had to rebuild the mod to make it work for more than 1 ball.
I'll first explain what all the mods have in common (1) and then explain the differences between them (2).
(1) What all the mods have in common
Each mod has 2 colored mats (red and blue) and yellow balls.
If a ball hits the blue mat --> red player wins.
If a ball hits the red mat --> blue player wins.
(Both the ball and the colored mat turn grey once they collide, see image below)
Less important but also noteworthy:
In each mod, players have slightly more bouncy hands, feet, elbows, and knees.
This is to encourage hitting balls cleanly since you get a little extra speed hitting them with an extra bouncy body part or joint.
In each mod, balls can cause a little damage if you don't hit them with hand, foot, elbow, or knee.
This too is to encourage players to hit balls cleanly but also to prevent too many matches ending in a draw.
(2) Differences between the mods
2balls_v3_fixed.tbm
gravity -20
two, rather small balls (size 0.3)
standard players (except hands, feet, elbows, and knees are slightly more bouncy)
wall in the middle
ghostspeed 105 (for standard ghostspeed do /set ghostspeed 100)
Mods with faster, stronger, lighter players:
2balls_v4_ng.tbm
gravity -30
two larger balls (size 0.5, mass 0.0005)
players have 50% less density, 50% more strength, and 50% more velocity (plus hands, feet, elbows, and knees are slightly more bouncy)
no wall in the middle
standard ghostspeed
2balls_v5.tbm
same as 2balls_v4_ng.tbm, except balls are heavier/slower (size 0.5, mass 0.0007). This is for players to have more time to react to the balls changing their direction and make overpowered openers impossible
4balls_v3_fixed.tbm
same as 2balls_v5, except there are not 2 but 4 of those balls (size 0.5, mass 0.0007)