Are you actually serious that you can't think of anything that can cause one planet to warm without causing other to follow suit? It seems like just 10 seconds of thought is all that is needed to think this through, so you must be trolling. Perhaps I am just falling for a subtle troll then.
Imagine this. . . Mars is approaching its perihelion (the point in its orbit where it is closest to the sun). Then Mars would get warmer, but if Earth was leaving its perihelion then it would be getting colder at the same time.
With quick thought you might also come up with volcano action, greenhouse effects, radioactive elements, bombardment by astroids, or world-wide wild fires. It isn't that complicated!