Most of the time I jungle with champions like Jarvan, Fiddle, Warwick, nunu or amumu. But the last few days I had to fight against 2 junglers. I didn't find anything of it, how they do it or how to counter it. I think one does red, the other one does blue and then one starts counter jungle and the other starts ganking.

However, as I have seen, double jungle has his problems. In the beginning it is really strong, but at the end the enemy team has no farm and lower level, most of the time, then the rest. So if you have a good begin game, and know how to react on the double jungle then it should be an easy win game.

This is what I would do, ofcourse I'm not such of a strategist myself. First and for all I would send out the support and AD carry to the red buff to defend it, for if one of the double junglers want to steal it (shaco for example). Then if the red and blue spawn, the jungler takes on blue and the support sacrifices one of his wards at the red buff.

As Jarvan, I fight wolves and right after blue and then I can go gank, so mostly I leave the red buff for a bit. This makes the possibility for the team to trap one of the junglers and down + mid should go and take him out, if jungler is there he should go to.

And for the rest I would say, try to survive it and then the double junglers will be nothing in the team. If anyone got good ideas to counter 2 enemy junglers, please tell me.

As long as you have good ward coverage at the entrances to your jungle, you should be able to counter the double-jungler tactic. You, as the jungler will get pretty screwed over, and underfarmed as a result, so to compensate, your 2v1 lane needs to completely dominate their lane. if you think you are going to get double-jungled, make sure you pick a jungler who can defend themselves 1v1. Mundo would be the obvious choice here, but Shyvana, Udyr and Malphite would also work well.

What is crucial here is the 2v1 lane. If that lane does not obliterate that one enemy champ. You will be in a lot of trouble. By Obliterate I mean that you will have to deny both gold and experience to that champ ensuring they are useless late game. Due to the fact that there are two junglers they too will have weak farm and be relatively weak later. The trick is to survive the initial OMGWTF of 2man ganks and counter jungling.

When this has happened to me in the past I have asked the support to follow the jungler. With the caveat that they still keep there eye firmly on the AD carry. Warding for him whenever its required.

But that's why most of the time they put an OP champ on solo lane vs 2. And even then the OP champ will play defensive (it's also mostly a range champ, so he can get gold from further). When the two others are standing high to catch him, the two junglers just come for a gank.

Unbelievable counter to Doublejungle: WARDS!
Ward you jungle entrance and ping you Mid, Top or Bot if one jungler enters you jungle.
You can kill him or make him wasting his Flash. etc.