Should a 2LT salute a 1LT?

Yes. While the salute may not be expected nor the requirement "enforced" in some units between a 2LT and 1LT, especially when other soldiers are not around, it is still proper to exchange the salute and should be done. Do LTCs salute COLs? (they are both just colonels).....do generals salute generals with more stars? (they are both generals). You betcha. A 2LT who fails to salute a 1LT in my presence WILL be corrected firmly.

What is the big deal. I suggest that all salute a 2nd LT in any combat zone

Saying a "Lieutenant is a Lieutenant" (for the Army, Air Force and Marines) is wrong. Should a Navy Ensign salute a Lieutenant J.G.? Is a "Lieutenant is a Lieutenant" true in the Navy? Is a Captain a Captain when speaking of an Army Officer to a Navy one? (Though I did have fun making reservations at the Naval bases while I was in the Army.) An O-1 should always salute any officer of a higher rank, no matter what that officers rank is.
As a Cadet Candidate at the US Military Academy Preparatory School (at Fort Monmouth NJ before its move to West Point) my Pay Grade was E-3. Depending on the Cadet Rank I held my insignia could look like a Lieutenant or Captain (I was never high enough up the food chain to have more than two bars) and would be saluted by NCOs and/or junior officers. We were taught to return the salute and don't make a big deal of it. In that case it was the impression not saluting could portray if enlisted soldiers did not receive a salute in return. As for the Officers, we were normally saluting them first, they just thought they were saluting us.

I'm astonished this question was ever asked...

Technically yes, but when I was a 2Lt someone told me that "rank among lieutenants is like virtue among whores; it doesn't exist."

Yes. First, it is simply a matter of respect, and earned, even if promotion to rank was "automatic." It is also important that others who may see the interaction understand the respect the officers show for each other. As career warriors move up through the system, the discipline learned early is very important.

I agree, an officer salutes another officer no matter what their rank. The lower rank does the first salute though. it is a sign of respect and that is how saluting started to begin with. Goes back to the time of Caesar in fact. The Roman soldiers saluted the officers and the officers saluted each other in signs of respect and to acknowledge the fact that the officer in charge was not surperior but a better leader. And this was usually due to better knowledge and education.

Dumb issue. Of course a 2nd LT should salute his/her senior officer. As I read it a 1st LT is still senior to a 2nd LT. I would go so far as to say that a 1st LT who is a platoon commander should salute his/her company commander even if the CO is of the same rank. It is showing curtesy to the rank/position of the senior person.
Of course in a private unofficial situation when there are no troops involved they may be as informal as the senior officer feels appropriate.

Yes, you need to salute any senior officer to you; even a 1LT. Always show respect to any senior officers; set the tempo for all junior officers; you will be respected by your enlisted folks! I use to be an enlisted NCO.

During my last summer at West Point in the late 50's, I was assigned to the 82nd Airborne Div. as part of the Army Orientation Training (AOT) program. I was assigned to a 1LT in the 2/504, and my job was to shadow him all day to learn how a junior officer should conduct himself and how to interact with enlisted personnel and officer of all ranks. One day, we were walking down the street in what was then called the "New" Division Area (now all gone and replaced by dorms). A 2LT was walking up the same street on the opposite side, and tried to pretend that he did not, or could not see my LT's rank. He diverted his eyes and attempted to get by without a salute, but my LT yelled at him to come over to him. When he got in front of him, my LT stood him up tall and chewed his ass something fierce for not saluting. He gave him a lecture right then and there on the need to show appropriate courtesy to all ranks, because it was the building block of the entire chain of command. I ;earned a valuable lesson that day, that respect for, and observance of, the customs and traditions of the Army, however trivial and unnecessary they may seem sometimes, is very important. It's the courtesy and tradition that counts, not whether you may think that there's no difference between a 1LT and a 2LT.