Re: Why Two Oaths of Office for President?

Chaz - I initially have found the similar results that you have come up with. But with a deeper search, I found Dr Eduardo Rivera's explanation below. Note, during my earlier studies of income tax research, Rivera's name I am previously familiar with. He was somewhat connected to Otto Skinner's work - "The Biggest Tax Loophole of All". If you click on the 'Source Link' it will take you to the full article where he explains the precedence that George Washington had set in place with his taking the Article II Oath of Office - it was for employees of the government - check it out.

Dr. Eduardo M. Rivera, Professor of Law & Government also agrees:

“[George] Washington took an oath of personal employment to "preserve, protect and defend the Constitution of the United States," for a government employer. Washington could have taken the Article VI Oath specifically provided for "all executive and judicial Officers, both of the United States and of the several States." Instead, Washington set the secret precedent that he and future Presidents would work for the Congress of the United States as its employees.

Washington’s agreement to work for Congress amounted to a false oath, because everyone not intimate with members of the First Congress and George Washington believed that the oath would bind George Washington "to support this Constitution." That mistaken belief that "this Constitution" had been adopted enslaved millions of Americans to a Constitution of no authority.”
-– Dr. Eduardo M. Rivera [Emphasis mine.]
SOURCE

Note: The significant verbiage to be noted is pointed out by Dr Eduardo M. Rivera. Specifically, the Article II Oath of Office (taken by every president since the time of Washington) lacks the notion that the president is sworn to "support" the Constitution.

On the contrary, the Article VI Oath says:

"I, [name], do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter. [So help me God.]"