If you own one of them, mix in LGPL code, and publish it, the result is entirely LGPL.
If you don’t own it and cannot legally relicense part as LGPL, you’re not allowed to publish it.
Just because you can merge someone else’s code does not mean you’re legally allowed to do so.
This is not correct; you're simply required to follow all applicable licenses at the same time. This may or may not be possible, but is in practice quite commonly done.
> Just because you can merge someone else’s code does not mean you’re legally allowed to do so.
> This may or may not be possible
I am not sure what you are saying, that is different from the comment you replied to.
Completely depends on how much you've "mixed in", and facts specific to that individual work.
Fair use doesn't get thrown out the window because GPL authors have a certain worldview.
Second, there are a lot of non-copyrightable components to source code - if you can't copyright it - you certainly can't GPL it. These can be copied freely by anyone at any time.