My opinion is that yes, changes DO cause stress, even GOOD changes. (I read a study once about humans who had terrible luck - such as a spouse dying, and great luck - such as winning the lottery. And the stress levels and consequences, such as heart attacks, etc, for the really good luck were surprisingly in line with stress levels for those with the really bad luck!) And that many stressful changes can EVENTUALLY become normal for the snake so that changing from THAT is stressful. For example, I believe that if 2 snakes have lived together for a long time successfully (don't ask what a "long time is - probably many, many months or years), then they have probably gotten very used to the cagemate and separating them will be TEMPORARILY stressful, just as changing to a different cage, substrate, or a new home is temporarily stressful.
If you receive 2 snakes that have been living together successfully for the past year and seem to be 100% healthy, my advice would probably be to try to change little or nothing for the first month. Try to keep them in the same cage, substrate, temps, etc, as long as nothing seems too absurd (way too hot or cold, etc). Then after a month of eating and digesting well (the most obvious signs of a thriving corn), it is time to separate them, but keep all other conditions similar to what they are used to. After several weeks of individually eating and digesting well, then it is time to make other changes, such as different substrate (if you don't like what was being used), more handling, etc. Then the changes come slowly instead of all at once.
Some might argue that the first month of cohabbing could bring a disaster, and that is true. But everything should be evaluated on a "likely cost vs. likely benefit" basis. And if they had been cohabbing successfully for a year, then my opinion would be that the likelihood of a disaster in the first month vs. likelihood of extra stress in the first month, weighs in with leaving them together while they adjust to their new home. After the first month, then they would get separated. And of course, if any signs of stress or other problems popped up during that cohabbing month (not eating, regurging, or ??), then they would get separated right away.
That is just my opinion - others may disagree. But it seems logical to me.