We have a closet with those type of doors and I just used it as an excuse to use some funky glass bureau-type knobs. Actually we have it in two places, now that I think of it. In the kitchen I used knobs that were white enamel in the center and trimmed with brushed chrome to match the other doorknobs (which are plain brushed chrome). For our mudroom closet, I used funky oversized amber glass drawer pulls from Anthropologie. Depending on your decor/style, you could go crazy or just match your regular doorknobs. Also, depending on the size of your doors, regular-size knobs to match the others might look too large.

We have a couple bi-fold doors in our home and I purchased cabinet door knobs that closely resembled our regular door knobs along with a matching plate to give the illusion that they were smaller versions of the full size door knobs.

The real question is do you want the doors to click closed with a latch or are you going to be happy with them sitting closed but not latched?
A cabinet knob is usually put on a door that does not need a catch to keep it closed. A regular door knob latches the two doors closed so that unless you turn and pull the knob the doors won't open.
IF I had animals or small kids in the house on a regular bases I'd want the latch to keep them out of the closet.

Our doors without the "normal" doorknobs have latch-type mechanisms in the floor, so they do stay closed.

We have dummy levers on the pantry French doors and dummy round knobs on the hallway French doors. The doors latch with a mechanism at the top of the door frame.

Thanks all for the thoughts. I would say our doors look pretty much like Natal's. Our also have a catch on the top of the doors so it wouldn't be a functioning door knob if we went that direction.

About five years ago, we put real french doors (with the glass panes) on one of our closets. I bought a modern pair of pulls for the doors. They are nickel and are actually large drawer pulls that have a serpentine curve to them, but I placed them vertically. I love the way they look. Oh, and we also put catches at the top.

I used dummy knobs.