I carry a knife on each side. Mainly because I like knives and this allows me to have twice as many to play with during the day.
If I'm carrying a gun (gun laws are VERY restrictive here), the knife in my left pocket will be something geared towards SD/gun retention, while the one in my right pocket will be almost purely utility oriented.
When I carry something expensive, fragile or more stylish than useful in my right pocket, I'll add a trusty workhorse on my left side.
I also enjoy carrying two of the same model (sometimes one plain and one serrated), a mini and full size version of the same model, etc.
I used to carry a single folder and a SAK, but now I feel weird if I don't have one on each side.
The only exceptions are when I'm outdoors (I switch to a single medium fixed blade and a SAK) or when I'm going somewhere with metal detectors or possible pat-down (even if legal, two or three knives might get a bit too much attention).