Two wearable items on back

[Kyviantre]

Shields really wouldn't make sense hitched up on a shoulder, but bows do. There already are shoulder-packs, so really they just need to make bows shoulder-worn instead of back-worn. And then you can sling your pack off one shoulder, have a bow on the other shoulder, and your shield on your back (where it might actually offer protection, which would be a nice bonus).

Umm...I always toted my roundshield over my shoulder.  You attach a loop of leather cord to the grip, then when you're done beating away the barbarians/zombies/necromancers/wardogs/natives (delete as appropriate), you just sling the loop over your shoulder.  Technically it is against your back (because there are very few people who can wear a roundshield and not have their entire back covered!), but it is still (also technically) being 'worn over shoulder'...like the below (huzzah for google images knowing what my random hopeful keywords meant!):