Question on Kong Double Ascender and Safety

[Dillweed]

Are ascenders like the Kong double or Petzl double safe to use without a prusik safety? I have a Kong Double Ascender and it makes long footlocks cake, but when I tie a safety prusik above it it binds and makes the advancement difficult. Difficult to where I'd rather not use it. It's not like when you advance a prusik with your hands. That slides up easily. The double ascender separates the ropes slightly, tightening the prusik.

Is it safe to use without a prusik? Are there any other safety measures available that are not too cumbersome?

thanks