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.
Originally Posted by Gerald C Koch
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.