Helper in M.U.G.E.N

I don't know how they to add a partner in M.U.G.E.N.

Examples of characters that I looked up

• Crow - Testament (Guilty Gear)
• Duck - Duck Hunt Dog
• Jake the Dog - Finn the Human (Madoldcrow1105)
• Mamahaha - Nakoruru
• R2-D2 - Luke Skywalker (DJ HANNIBALROYCE)