This deals with Unhinged cards, so competitive REL is useless here. Imagine that I have a Gleemax on the field. It says:
You choose all targets for all spells and abilities.
Destroy target silver-bordered permanent in any game you can see from your seat.
Real Answer
Unhinged cards bend the rules. You and your friends should decide how to handle the situation.
How I Would Settle It
If someone in another game gives you an Ass Whuppin', there's nothing that Gleemax can do about it.
Gleemax must to be referring to spells and abilities in your current game. Otherwise, you would have to choose all targets for all spells and abilities for every game in the entire world. That's impossible to do in a timely manner. Also, by the time Ass Whuppin' crosses over between games, it's too late! It has already been cast, and the targets have already been chosen.
Multiple Gleemaxs follow timestamp rules. The controller of latest played Gleemax gets to choose all targets for spells and abilities.
As the other answers say do what ever your play group think is most fun. That being said I would say that if you control Gleemax and an Ass Whuppin' is being cast on a card in your game you get to choose the target of the Ass Whuppin'.
This is based on a few things that Mark Rosewater (Head Designer of Magic, and Silver Bordered Rules Manager) has said on his Tumblr. He has said that players in either the originating game or the targeted game can counter the Ass Whuppin' he has also said that if a player from the originating game counters it a player in the targeted game can counter the counter. From this we can gather that Ass Whuppin' can somehow merge the stacks of 2 separate games temporarily, because of this fact I see no reason for Gleemax to not effect spells on the stack in its game.
As for multiple Gleemaxes there are a few ways to rule it. One would be having the effects go by time stamp order, so whoever played the second one would end up choosing targets. You could have it act like a replacement effect (the wording isn't exactly right for it to be a replacement effect, but templating on Un-cards isn't the best anyway) in this case whoever casts the spell would decide what order to apply the redirection effects (probably applying theirs last and getting to choose their own targets) or there is Rosewater's method:
Here’s what I recommend. Active player goes first and removes a legal target. The other player then goes and removes a legal target. You keep going back and forth until only one legal target remains. That’s the target.
I would say for simplicity that Gleemax only affects an Ass Whuppin' originating from your game.
The problem is there isn't a true answer to the question as neither card works within the magic rules framework.
Ass Whuppin' bridges games temporarily, does that mean it goes onto the target games stack or the original games stack, or some version of both?
Gleemax doesn't explicitly restrict you from backing out of a spell cast and since paying costs is done after target selection it is trivial to just back out of a spell that is targeted like you don't like.
Luckily both these problems are resolved by being silver bordered, the answer is "do whatever everyone agrees to". Decide ahead of time how to resolve these kinds of situations and stick to it. Note I mean something general like using a judge or by democratic vote.
