I have also read some of Bernhard's threads on this. However Bernhard also makes the point that you don't put anything HT until HS is complete. So should one initially making sure the whole work is safe and secure and up to speed HS and then do the sight reading through to isolate the difficult HT bits to start on?
Er..

that is not exactly what I said.
There is no need to learn a piece with hands separate first and then and only then join hands (except in the case of counterpoint music where this is a very good idea).
You need to learn hands separate only the passages for which you have not figured out yet the best technique (meaning movement patterns and fingering). Although a beginner may have to work on the first pieces hands separate (simply because s/he has no technique), soon s/he would be doing most of the piece HT straight away and doing HS only on selected sections. And advanced pianists may never need to do hands separate at all. Hands separate is a must only if you do not have the technique.
Sight-reading on the other hand is a different department altogether. When sight reading one should always do it HT.
So, as everyone said, this is how you figure out the most difficult bits: through sight reading.
However sight-reading (useful and important as it may be) is a terrible strategy to
learn and practise a new piece.
Sight-reading, HS, HT, there is no contradiction. Each is important, each has its place and each must be used correctly. It is really like baking a cake. It is not enough to have the ingredient. They must be in the correct quantities, and they must be mixed in the correct order. And the oven must be at the right temperature and the cake must stay there for the correct amount of time.

Best wishes,
Berhnard.