Flight suits, based on what I could find, are by definition, one piece.
The CRL for ESB Fett ( for example) calls for a flight suit, and gives details about the sleeves, pouches , etc. It doesn't say that you can substitute pants and a shirt. I'm sure that you could, if there were a medical condition of some kind. As a GML, I would only approve those that had a one piece suit, unless there a special circumstance. If a flight suit isn't comfortable, this may not be the costume for you. I'm overweight, and while the costume has gotten a bit snug, it's still comfortable. This has prompted much needed bike riding and food plan change to better be a Fett!
As Stormtrooperguy has said, don't stray from accuracy.