I do not suggest doing that you know what's coming and they don't, and you know where you want the story to lead, it could seriously be not only difficult but unfair as well, and you should try to enjoy yourself with NPCs

You'd only know if the alternate gm was running your story, or vice versa. There's no conflict if the alternate gm is running a side story of their own creation that's completely separate from yours. Two people can tell completely separate stories that involve the same characters within the same universe ya know.

I do not suggest doing that you know what's coming and they don't, and you know where you want the story to lead, it could seriously be not only difficult but unfair as well, and you should try to enjoy yourself with NPCs

You'd only know if the alternate gm was running your story, or vice versa. There's no conflict if the alternate gm is running a side story of their own creation that's completely separate from yours. Two people can tell completely separate stories that involve the same characters within the same universe ya know.

I think EmptyCratesOfDeathSticks meant situation when GM has own PC active when he is GMing. In rotating GM situation you are fully correct (unless GMs are coordinating the story together, which again is situation which can be solved).

I make up my NPCs(friendly and enemy) so that they can be played with. So far I've only gotten to be Duros Pilot with them but it's been good. As long as they don't feel like you're stealing the show them I think it's OK.

I think the end result is, YMMV. I have been running a game for almost 2 years now where I have an active NPC/PC. My wife and I swap GM'ing duties and are slowly coaxing the other 2 players into GM'img (other things at first, but...). In our case, there hasn't been a single instance where it has been a problem, none. But between the 2 of us we have over half a century of gaming. Never even crosses either of out minds to have our characters outshine the others. When we are running, our characters are holding the go-pro, focusing on the others in the group. That's not to say our characters don't shine in their specialty areas, but the focus is on the other players\characters and the group as a whole. We have a very candid group (sometimes TOO candid, lol) so if this became an issue, it WOULD be addressed.

Biggest recommendation I can give, know your group/players and know yourself. If you plan on doing so, and immediately think of your character first, just stop. Have him/her/it take a holiday or perform a "side mission" that has no real effect on the adventure but is otherwise "necessary". Thoughts should be on the group and adventure first.