The ability to do this is one of the impossible things that the Work attempts to convey. In fact, let me point out that I have other explanations than the following, which is good also. But something just occurred to me when reading this discussion of Ecclesiastes in Wikipedia:
What occurred to me is that the ability to think two things at once, is only contradictory at the level of binary thought, AND-- you could think of the ability to think two things at the same time, as an alphabet block with different letters on each side. Thinking two thoughts at the same time is then just like looking at such a block from a certain angle: an angle which includes multiple sides of the block. To paraphrase somebody else, if six impossible things before breakfast is too hard, try thinking twelve, before lunch.
Some passages of Ecclesiastes seem to contradict other portions of the Old Testament, and even itself. One suggestion for resolving the contradictions is to read the book as the record of Koheleth's quest for knowledge: opposing judgements (e.g. the dead are better off than the living (4:2) vs. a living dog is better off than a dead lion (9:4)) are therefore provisional....