It is rumored that during Russo-Japanese war there was serious conflit between those two after Battle of Liaoyang and that Samsonov even slapped Rennenkampf over his face.

But main sources of this rumor - former German general Max Hoffmann, who was military observer during Russo-Japanese war and Russian novelist Valentin Pikul. Some novelists even transformed slapping into whipping.

But Hoffmann never witnessed said slapping himself, actually never met Samsonov and Rennenkampf during Russo-Japanese war, and there's no any confirmation except Hoffmann's words.

It's safe to assume that mutual despise between those Russian Generals is merely an unconfirmed rumor.

The two's poor relationship was a result of the Battle of Mukden, a disastrous defeat for the Russians after which Samsonov publicly claimed that von Rennenkampf had failed to support him during the battle. That was the genesis of the enmity between the two.