This does not require 2 people. When you start up Campaign, choose co-op. Play the first mission and do a triple combo with one character and quickly switch characters with your left hand and do one more super attack.
Once you hit a 4x Combo this will automatically unlock.
Apparently you can do this in Co-op Arcade Mode as well with two characters you feel comfortable with using/switching between to combo their Super Attacks.
Thx to Munoz2024 and bozack for the helpful tips.