Historically, we know what motivated Constantine. He was a Roman Emperor.
He had to think about the unity of the Empire.
You have been misinformed.
First you need to learn more about this divisions and then you will be able to talk about unity
Diocletian just splitted the military and civil responsibilities, so that any crises could be handled right away. Each co-emperor would have his own army, bureaucracy, and so on. They were still all interrelated, though, and the senior of the two Augusti would be the one to set general policy.
Diocletian made the decision to subdivide the empire, because of the crisis of the 3rd century revealed problems that didn't exist before, or were there, but not noticed or addressed. But to say he simply split the empire is inaccurate
He actually made several changes to the imperial office, such that the result is called the 'dominate', rather than the older 'principate'. In many ways those changes were merely evolutions of preexisting trends, whereas others were innovations of his (the subdivision and succession systems were in the latter).