Complex sentence: Since his reign was brief, and he may never have been more than co-regent, the evidence for Smenkhkare is not plentiful, but nor is it quite as insubstantial as it is sometimes made out to be.
Split into simpler sentences: Since his reign was brief, and that he may never have been more than co-regent, the evidence for Smenkhkare is not plentiful. But nor is it quite as insubstantial as it is sometimes made out to be.