Is it permissible to combine and shorten the Dhuhr and Asr prayers into a single prayer with one Taslim and one Tashahhud? Is there a difference between them with regard to Tashahhud? And is the Fatiha and a Surah recited in every Rakah when combining?

It is not permissible to combine two obligatory prayers with a single intention, a single tashahhud (definition), and a single taslim (definition). Rather, each prayer must be separated from the other by a taslim, as if praying each at the time of the other.