Nick Offerman goes to his father's funeral and discovers he has a brother from his father's secret "other" family: Zach Galifianakis. Nick's whole square world comes unglued. Zach is a professional force of destruction.
I don't care what else happens. Just put these guys together.
What do you think?
You've got some good ingredients but the wrong recipe here. Zach attends his father's funeral and the video "last will" expressly and hilariously states (through the visage of some old-hollywood manly leading man type) that in order to prove he isn't entirely useless, Zach has to go find Offerman's character (the son he had by an adventurer/hunter/outdoorswoman while on Safari [who if she appears can be played by Julie Andrews or the like and carry off a few scenes by being posh, old and English while wielding a hunting rifle.]) and together they have to conquer some survivalist challenge or hunt some notoriously elusive animal and if successful will be rewarded with half of his fortune each. If Zach comes back empty handed, he gets nothing.
Zach finds Offerman's character in Alaska or South America or whatever being manly and outdoorsy and exactly like his father always wanted Zach to be, Nick wants nothing to do with this soft rich guy who knows nothing about any of it.
Through the trials and tribulations of their task, they grow to like and then love each other, eventually bonding and accomplishing the task but deciding Zach is better off not taking his father's spiteful money and coming to stay with Offerman on his adventures (because it turns out he is already rich and never needed the money).
Nick might also find a love interest along the way, while Offerman surprises with a series of humorously implied one night stands with both men and women (and possibly a lion).