The only reason I can think of would be because of our going into Iraq with out permission from the UN. If we did it there we would have no problem doing it again and Russia knows it.
So the only reason for them to cooperate is to keep a foot in the door of Iran, to have a say in the Mid East. China is the real question, our going into Iran has even larger implications than it does for Russia.
Neither China nor Russia wants us with a foothold in Central Europe, true. But I can't think of a course of conduct more likely to draw us in than what they have pursued.
I doubt whether we have ambitions to occupy Iran, however, even for the short term. So maybe their best option would be to get rid of the mad mullahs and let us turn our attention elsewhere.
Both countries are within range of Iranian missiles, and subject to attacks by Muslim terrorists. I wish they would stop playing with fire.