Mind if I ask you a question?
If I give my Pokemon a Power Belt and go into double-battle grass, upon killing two Swaloons, will my Pokemon gain 4(Power Belt)+2(Swaloon)+2(Swaloon) for a total of 8 DEF EVs, or will I get 4(Power Belt)+2(Swaloon)+4(Power Belt)+2(Swaloon) for a total of 12 EVs?