EVERYONE carried both, or even more. The most I've personally seen carried a fixed blade on their harness/plate carrier on the left shoulder upside down, to draw with their left hand, and a reasonably large sized folder somewhere on their body, often their right pants pocket, or cargo pocket.
Most men I've served with also had a SAK or Multi-tool for the screw-driver, saw, belt cutter and other applications. Thats 3 knives in total. Some people chose to carry a "boot knife" kind of small stiletto, if they were trained in killing with those. A stiletto like knife can be a perfect weapon in some cases, but you have to know exactly how to kill with it to be effective, because knife fighting is an art in itself.
Knives, folders and SAK/Multi-tools give you more functions then any single fixed blade, but a stiletto-type knife worn in the right place offers you a kind of killing power none of those other types give you, therefore its in a lot of commando style units emblems.
Once you experienced the power of these in the field you won't leave base without it, but the first times you use it are very awkward, unless you're a certifiable psychopath. Training helps a lot, until you gut an enemy for the first time. After that you know if you want to carry one or never use one again.
I stick with a Gerber LMF MKII, a Leatherman Supertool 300 and a Wilkinson Sword Fairbairn Sykes F~S type in my boot. The latter i hope never to use again.
In my police/swat type role these days I stick with the Leatherman only.