I talk in reference to recent opinions on Mercedes and long standing opinions on Red Bull/Ferrari. In recent history it does seem the clear and focused number 1 and number 2 driver works a lot better.
Take the Ferrari era in the early to mid 00's. Schumi and Rubens were far from best friends, they by most accounts were cordial for the most part until they fell out in America in 2005. But, they delivered as a tandem, Schumi was the clear number 1, and like Irvine before him seemed to accept that role for the most part (unlike his Brawn days). Everyone knew their role and their was little in fighting. The results speak for themselves.
Vettel and Webber have been the same since 2010 to a lesser extent. Webber has shown more open discontentment, and has cried foul openly a lot more, but a lot of the time his number 2 status has been down to Vettel outperforming the aging Aussie. With Vettel the number 1 priority, Red Bull have reaped rewards.
2007 proves that if you mix top top drivers together things can go haywire. Lewis and Alonso for various reasons did not deliver a WDC or WCC to the team with arguably the best car. Think about that for a second, the two best drivers in the field with one of the best cars could not bring home either title. Yes, one of these failures were due to spygate, but the i-fighting and controversy could not have helped. The following year in a car that I felt was inferior to the Ferrari, Lewis and McLaren with Heiki as a clear number secured the WDC.
People point to Senna/Prost, but their car was so far leaps and bounds ahead of the rest of the field (and their driving talents were to be fair) that it was hardly a team effort as there was no team really there in 1989 when things went sour.
This is why as much as we can hope we get Vettel/Kimi or even better Vettel/Alonso we cannot blame Red Bull if they choose the underdevloped Riccardo as the driver to support Vettel. I believe Riccardo if he does go to Red Bull next year will become like Heiki was to Lewis in 2008/2009. Which for Vettel and Red Bull is no bad thing.
Kimi despite the friendship maybe a bit too much of a shall we say character to stand for being number 2, and if Kimi starts to outperform Seb I cannot see Seb taking a back seat. I think it could end up like Lewis/Alonso with Kimi riding off to the sunset after a year or so. I prefer Alonso to Seb as a driver and as a person, but Seb was right in saying Alonso is not someone you would want as a teammate and not just because he is a great driver.
Lewis and Rosberg is going the same way as Vettel/Webber it would seem. As Lewis has adapted to his new car/surroundings he has modestly outperformed Nico since Canada, mostly on merit. I was watching a old race on Sky from 1991 with Mansell and Patrese, and it seemed like their head to head was similar to Lewis/Rosberg. Mansell like Lewis took time to adapt and Patrese outperformed him like Rosberg did Lewis. Once Mansell found his feet, it was back with the program with Mansell as the number 1 like contracted and Patrese the number 2.
I am not saying Lewis is contracted to be number 1, but it may benefit the team if they focus the car around him and put him as the clear number 1, which people are claiming they are already doing. A clear focus on one driver and a steady number 2 to support as shown above does seem to work. If Rosberg is a bit too good to be number 2, there are other teams out there for him I am sure.
So in short, I think it is best to have a clear food chain with teammates and not two bulls battling it out (no pun intended) I hope we get Seb/Kimi or Seb/Alonso in the future, but as shown above there are perfectly rational reasons why it should not happen if history teaches us anything. If a top driver can get their own version of Rubens, Massa post crash or a less whiny version of Webber, I would advise them to do so.