As far as I can tell the level of the quest (bronze, silver, gold, platinum) is effected by both heroes. If I have both characters at about halfgreen focus the bar seems to be triple arrow up which is faster than if I have one hero in the quest.
Also - Taxes at levels 1 to 4 seem to be $100, level 5 to 9 is $250, and level 10 is $500.
Also the watcher's pavilion, kingball court, lighthouse and mill never cost more than 30 RP each.