As for the first question about bonuses Runesmith's ability requires you to pay 2 resources to have target unit gain 1 power icon until the end of turn, and can only be triggered when Runesmith is in the Quest zone.
You would not get a second bonus from a second Runesmith in play for only paying the 2 resources to activate the action of a single Runesmith.
Example: you have 2 Runesmiths in your Quest zone, you designate Runesmith 1 as triggering his Action paying 2 resources. Runesmith 2 does not contribute anything in this example as he has not triggered his Action.
However, since there is no limit printed on the card, and this game does not use a mechanic such as tapping to prevent the same unit/support/development from activating an ability a second time, you would be able to pay the cost of 2 resources any number of times designating either of the Runesmiths as the unit completing the action for each Action that was triggered.
A card such as Defend the border: (While defend the border has 3 or more resource tokens on it, redirect the first point of damage done to your capital each turn to another target unit or capital); would also not benefit from two copies in play with 3 tokens each on them as it only redirects the first damage.
However, If you had a card such as Keystone Forge in your Kingdom zone, (Kingdom, Forced: After your turn begins, heal 1 damage to your capital) would benefit from having 2 or more copies in play as each's Forced action is triggered from you beginning your turn and does not specify a specific damage to heal (as Defend the border designates the first point of damage each turn to redirect).