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.