You can run up to 3' on a ground and if two seperate amps they dont have to be same length. Now if strapping them yes the need to be same length due to acting as a Parallel run then refer to Ohms law and Kerkoves laws of electrical theory on why.But again ONLY if your not extending either of those ground wires past say a foot or a foot and a half to get there. and be sure to sand down the paint around the bolt to nice shiny metal, as that should be your actual ground not the bolt that holds the wires secure