Still the storm raged furiously. Lightning, thunder...winds gusting strongly, enough to bend the searchers over double.
In this image, the orange cord is bent over double. (It is bent or folded back upon itself.)
To bend means to fold.
To double (as in double up) means to fold something in two, or to fold something in half.
To bend something over double. means to fold something over upon itself. LIke the orange cord in the image.
You bend a sheet over double by folding the sheet in half and holding the sheet by the two edges and letting the rest hang down toward the floor. Thus the sheet is bent over double, or doubled over on itself.
In the case of a body, you bend one's body so as to bring distant parts into proximity (my OED definition of double up (8b) which gives the same idea.)
So for the searchers: the wind gusts were strong enough to fold the searchers over upon themselves or to bend the searchers over double. Each searcher is now doubled over upon him or herself. The exact meaning of the phrase, as used in the passage, is not known. Is it figurative? Literal? Does the wind bend the searchers over double in a general sense? Does the wind snap each searcher's back and bend the searcher over double backwards? Or do the searchers, facing the wind, bend themselves over double, so as to shield themselves from the wind? Even in the latter case, it can be said that the wind is the agent of the searchers' bending over double.
For a person to bend over double, it does not necessarily mean that the person is bending all the way down, back and knees straight, and touching their toes.
In general one bends over double by bending at the waist and leaning the torso forward over the body, And it can include a posture with the hands or arms resting on one's knees.
Think: (1) a person runs 10 kilometers and then stops and bends over double, hands on knees, to catch their breath. (2) a person gets bad news in a doctor's office and bends over double, crying, so that the tears fall to the ground. (3) A person hears a joke and bends over double, laughing. These examples can all be found by contemporary authors using a database such as google books. But in none of these three examples is a person touching their toes.
In the passage, the phrase to bend the searchers over double can be read in the transitive sense: the gusts of wind were string enough to fold each searcher back upon himself.
Another possible interpretation is that gusts of wind were so strong that they caused the searchers (in the intransitive or reflexive sense) to bend over double, so as to make headway against the gusts, or shield themselves from the gusts, their face from flying debris, etc. They were bent over double, that is bending deeply from the waist, torso leaning forward over their body, but they were not touching their toes.
(To be bent over double can mean to be bending at the waist, knees straight and touching one's toes. Except, in my mind, that posture does not fit the description of the passage at hand, unless it means that the wind was able to bend the searchers over double in a figurative sense.)