And why not?

Two friends go in against each other head-to-head: 1) Okay, you kill me 15 times to close your handgun bounty, 2) Now Kill me X-times with the shotgun, 3) Now Mele... 4) etc. 5) etc. 6) etc. Now we reverse roles and both leave with most of our crucible bounties completed...(we good players now! Like the ToO lag switchers but didn't even have to pay for any equipment)... I would love to be able to call-out a Guardian one-on-one, but if that were possible, it should only be for bragging rights and none of the kills should count towards anything...not even statistics: Okay, we each create a new username so we can each create dummy characters. Then you kill my dummy 100 times without dying and I do the same to yours. In no time at all we could have the two highest crucible K/D ratios in the game without ever actually having to fight a single battle... Some people will brag and take great pride in the fact that they cheated their way to the lighthouse, but that just makes going to the lighthouse a little bit less meaningful every time it happens. If a "dual" mode is created, it should be done in a manner that is not subject to abuse by the blaming cheating idiots....

And why can't that be done on 2v2?

You cannot pick your opponents to ensure that you are fighting a friend that will collaborate in such a manner. I was specific about being able to challenge a specific person to a dual, where you would inherently be picking your opponent...

Then they can make 1v1 matchmaking so you can't chose your opponent and reward the winning player. & add an invite mode that lets you chose at the loss of your reward chance. They could make 1v1 rewarding.