In the script, the blade is a "triple pendulum" that swings down from the ceiling, rather than a blade that slices horizontally. There's no good indication why the VFX team turned this into a head-height spinning blade (and then added a secondary floor blade) other than that it might not be sufficiently clear why someone couldn't just crawl through.
The cobwebs begin to move...
INDY: He kneels before God. (to himself; suddenly) Kneel!!
We hear the awful rush of air -- WHOOSH! Because Indy is in
the act of kneeling, only his hat is knocked off and his
hair flies in his face.
Instinctively he ROLLS FORWARD on the ground out of harm’s
way and looks up. From his new position he is able to see
what is causing all the trouble: A RAZOR SHARP TRIPLE
Indy gets cautiously to his feet. Now he sees the pendulum
has been guarding a SMALL CORRIDOR which turns a corner to
the left fifty yards ahead. Wooden wheels turn -- the
mechanism controlling the spinning blades. Indy loops a rope
around the wheels, Jamming the mechanism and stopping the
blades in mid-swipe.
The second blade is absent in the official novelisation
He set his foot down and fell to his knees. As he did so, he heard a
loud whooshing overhead, and he instinctively tumbled forward. He lay
there on his stomach a moment, then slowly rolled over. He peered up,
and now he could see it above him—a razor-sharp triple pendulum, and
it was still whirring just inches over him. The pendulum was attached
to a pair of wooden wheels connected to the inside of the stone arch.
It was probably activated by the slightest breath of air created by a
person’s movement, and stopped after it struck its target.
and the graphic novel
I've seen chatter on the internet that this additional blade would prevent a Muslim from crossing into the chamber because they show penitence by bowing rather than simply kneeling, but I've yet to find any actual quotes from the film's makers to confirm this.