Dualwield doesn't work on spells. For mages, dualwield is just more magic stat thanks to 2x rods. There's two "broken" spells that work with dualwield, Sunbeam and Wild Card, and I don't know if those are bugs that Gumi just doesn't bother fixing, or intentional, but every other magic attack will not work with DW.
Hero rings are good for mages, and physical units with more than 150 base atk. They're far from best in slot, with even a lot of event gear or star quartz items being superior, but hero rings are still a very easy to obtain starter item for units. May as well craft some for when you do unlock the later tiers, since they're needed in the recipes anyway. I wouldn't go out of your way to farm for them or anything at this stage in the game.