According to Endelyon (who is the only other bard I know who has tried this feat) you gain an additional +1 to every inspiration (so +5 Courage becomes +6 with this feat, and +7 with the 1st level spell; +8 Inspire Competence becomes +9, etc).
The second part of the feat, which you were wondering about, is that you can effectively have two inspirations active: One of Inspire Slowing or Inspire Jarring, and any other inspiration of your choice. Whether this works, or how to activate it, is beyond my current knowledge of how the feat operates. Maybe you could try turning them both on at the same time, or maybe there's some new feat or combat toggle to this effect?
