up high , hanging the units? or down low?
she is in the bathroom! its a linen closet :)
I would only use two sets. I've used two sets on a butlers pull out that was filled mostly with canned goods...very heavy, and works fine. It takes some good layout to get two sets lined up perfectly. If one set is slightly off, the sliding can be labored, and cause unnecessary wear to the slides.
Most production cabinetry only uses one set of slides at the bottom and a guide slide at the top. In my own kitchen, I have my trash drawer and my dog food drawer as a short sided drawer box, but with a tall drawer front. I use only one set of slides, and the drawers work great. The drawer front lines up perfectly when closed.
There is a logistics consideration. You can mount one set at or near the bottom, and the second set separated from the first from 12" or more. The top set will act as a stabilizer guide besides carrying the weight, as it helps prevent the rocking due to top heavy pull outs.
Having them separated also allows the slide out's removal, as most full extension slides have the little lever that has to depress to remove the pull out. If the slides are separated, the pull out can be extended to where it stops, and the bottom lever depressed releasing that slide. While holding the bottom there, the top can be rocked slightly forward for those to be released.
To reinstall the pull out, set the edge of the lower drawer member slide into the cabinet member, and then tip the pull out enough to get the upper slide to get mated into its cabinet member. Another tip, most full extension slides have a by-pass in the movement distance of the extension part of the slide. When the slide gets inserted it will extend out initially better if at first it is shut closed briskly. That will set both slides to the near same by-pass points so they will work in tandem.