Board Thread:Destiny Forum/@comment-24280190-20131216163449/@comment-2601:602:80:2561:1CFF:F14A:4751:618C-20171219195945

Bungie had a dual wield easter egg in Marathon back in the day where the equivalent to pairing a hand cannon with a sidearm would enable dual wield by pressing both a modifier and "toggle weapon" key simultaneously. So somewhere in Bungie's old libraries is a way to do it.