I'm in the middle of reading Service With The Signal Corps: The Civil War Memoir of Captain Louis R. Fortescue. It's fascinating and great, but I'm constantly curious about the encounters he has with enemy soldiers while away from the battlefield.
As a member of the Union Signal Corps, Fortescue is often in a group of 2-6 Union soldiers with lots of autonomy, roving to various vantage points to set up lookouts. As he travels around/through the countryside, he's very careful to avoid Rebel soldiers and he's chased/attacked at various points by them.
But then in one instance (pg. 123-124), he returns to sleep at the home of a Union-sympathizing civilian in MD, but "the society he [the civilian] was enjoying, I learned, was not the most agreeable, there being two wounded Johnnies [Confederate soldiers] attended by two well ones on parole staying with him, but a good bed in the house of a good Union man was awaiting me, and rather than forsake that luxury on account of his unpolished guests, I remained."
I'm aware of instances where fighting forces would call a truce and interact while burying their dead, etc, and Fortescue discusses numerous anecdotes of Union soldiers staying in the homes of Confederate sympathizers (and vice versa happening in this instance), I'm assuming because of some kind of code of ethics between soldiers and civilians, regardless of their allegiances... but was it common for Union and Confederate soldiers to not attack each other in a situation like the one described by Fortescue? Did they have some kind of gentlemen's agreement to not attack while they were spending the night in a civilian's house? I'm so curious about their "rules" or etiquette in these situations.
In this case some language of the time is helpful.
Throughout most of the war, but especially early in the war, neither the Union or the Confederate military had the means to imprison the thousands of soldiers they captured. To accommodate the problem both governments would let prisoners go as long as they promised not to take up arms again for a certain amount of time. Typically this was done fairly quickly after a battle so that the parolees had a fairly easy time of getting back to their army and so the capturing army didn't have to take care of them very long. It was also usually stipulated that they could return to fighting if the army released a prisoner of equivalent rank back under the same system. All of this generally operated on the honor system and was rife with exploitation. After 1863 the system generally ended due to abuses on both sides eroding any trust left as well as a change of war policy and hardening of political aims in both the Confederacy and Union.
The Confederate soldiers at the house are described as "on parole," so in this case a Union soldier should, in theory, have nothing to fear from a Confederate parolees. They are non-combatants in a legal sense.
As to other examples, none come to mind of Confederate and Union soldiers interacting in a non-combat scenario unless one is the prisoner of the other. Any code of ethics or understanding that may have existed early in the war was certainly gone after early 1863. In any other scenario I'd expect them to fight or flee from one another. Staying in the house of a sympathizer of your enemy would generally fall under quartering, a questionably legal practice but one difficult to resist in a time of war. There was little honor or respect given or expected in this scenario. Soldiers often abused or stole from their hosts, and hosts might run off and warn enemy soldiers or violent possess about the men sleeping in their house.