Quote Originally Posted by Gerald C Koch View Post
Developing agents like hydroquinone which are lighter than the inorganic chemicals will settle out prefentially... The difference might be small but you cannot be really sure.
Yes the different components settle differently. Who knows what will be in what you mix if you do half. The mixed result may not have sufficent "preservative" so it won't last as long.

The worst case I can imagine is the results will be disappointing, causing you to be frustrated and making you feel bad about the whole experience.

That would be a very bad outcome, and wanting to make every amateur a success may be why the advice to "mix it all up at once" is so often stressed.