The x2 is purely for bumping the match bonuses up higher. It doesn't affect the rate that the players gain skills or the team training level.

There are some managers who get away with using low-condition players in their match squad, and others who insist on having one at peak condition (all 99%). Both scenarios work, which is typical Nordeus. There should be some obvious penalty to using a poorly-conditioned squad, over and above the theoretical liability to injury that is touted and rarely seen...