[etiam]

The reason two is harder is a bit subtle.

Hitting the population with multiple mechanisms simultaneously masks the selective advantage of slightly better phenotypes against the individual attacks.

There's a spread in individual characteristics, and if the attacks are applied independently, the more vulnerable individuals perish more readily and the genetic distribution goes on to trying states mainly near the more resilient ones against that attack. But faring a bit better against Attack-1 is practically no better than not, if Attack-2 is always going to wipe out all Attack-1-resistance phenotypes about equally anyway.

Executed just right it attacks the conditions for gradual evolution by breaking most of the corrective signal.