Silent Gliss two prong hooks for pleated curtains which use heading tape, nylon pleat hooks.
These small pleat hooks are often found on smaller installations such as boats, ships, caravans, motor homes, minibuses, trucks etc
NOTE sold in quantities of 1, photo for illustration.
Several other companies make similar double hooks for pleats, although they are all slightly different. The photos below show comparisons from different angles.
HINNO Double Fingered Pleat Hook comparison
There is one more variant that I know of, though it is slightly larger than those in the comparisons and that is the Regency Pleat Curtain Hook