I haven't reached a second skill/perk with any of my crew yet, so I'd like to ask some questions:
How is the reduction handled if there's insufficient XP in the secondary skill to pay for the loss? Is the current skill lost or is it kept and you have to retrain it to get rid of it (necessitating another loss if not paying with gold)?
What happens if you max out Skill A (eg Mentor), start training Skill B (eg Sixth Sense), then re-train Skill A to Skill B (Mentor to Sixth Sense)? Is the XP totalled for checking the training level of Skill A before getting to repick Skill B, or does the system not permit you to do this?
For interest, if the formula on the wiki is correct, by my calculations your second perk/skill needs to be at 38% to not lose any XP from the first skill/perk.
Incidentally, could someone point me at a refresher course on differentiation? I had to brute force the 'total xp' earned value through Excel as I couldn't remember how to differentiate the XP equation.