On the general swapping etiquette - Yes to let them know and yes to the fact that if you swap that should be one swap for the whole flight (not half half, visit each other etc).

Having said that you might also want to consider whether that might create issues for you with your company., The question of seat swaps comes up frequently in the BA forum (not so much on the AF one) and there, some people have reported significant issues when work paid for J and they swapped with a Y seat bought personally for a partner (in short, they were told this constituted misconduct). If your company is ok with this great, but if they are not, that might be a big risk to take depending on circumstances.