I suppose you could make a hole in two of the boxes.

Put the two boxes together at the hole and put one apple in the hole.

You could say half the apple is in one box and half in the other,

so that the fractions don't work out,

or you could say (since you can take the in-between apple is in 2 boxes)

one box contains 9 instead of 8.5, another contains 6 and the last contains 2.

Alternatively, I can loan you an apple.

Then you can put 9 in one box, 6 in another and 2 in the third.

You can put the apple I loaned you in your pocket.