They were allies by necessity, but I don't think there were a lot of illusions about where things age heading. After all, the official doctrine of the Communist Party has always been that every non-communist regime has to be violently overthrown and replaced by a communist one. USSR didn't have the bomb not because they didn't want one, but because they were incapable of building one by themselves, and stealing all the details by means of vast spy network they had in the US, and then recreating them on their side, took time. If they had the capacity, they'd do it as fast as possible.