TONDA Battery Terminals, 0/4/8/10 Gauge Battery Terminal Connectors with Shims and Protective Covers, Car Cable Ends, 1 pair

Good enough: Take negative terminal off battery, loosen those two bolts on it, shove second wire in the same hole where one wire already is, tighten bolts, put terminal back on battery. Best: Swap out your terminals for this style, which give you nice separate set screws for each cable: https://a.co/d/04cRCNBY