Twister is a steaming pile of Hollywood horseshit. Almost nothing depicted in this film is realistic.
First to the metal pipe.
We can realistically estimate the amount of force someone will experience in a F5 tornado. The free fall terminal velocity of a human being is approximately 55 m/s (120 mph, 200 km/h). This means that the wind speed has exactly enough force to lift the weight of the human body. Now an F5 starts with 116 m/s (260 mph, 420 km/h) which is the nearly exactly two times the terminal velocity speed. Unfortunately for our daring heroes wind force squares with linearly increasing speed, so the actual force quadruples.
If Bill weighs 1000 N, he is pulled with a force of 4000 N (!). And not with a securely fitting harness used on the thigh like a climber which can actually hold this amount of force, but with an improvised leather strap around the upper torso. If the heroes don't get ripped out immediately, they wish they were because the leather will tear off the muscles and ligaments.
What is also wrong is the convenient floating: Tornado wind is highly turbulent, they will be flung around like laundry (Which is the reason why I did not use the higher fall velocity for being head-on; They cannot hold that position). But the pipe does not even budge or bend which is strange when someone realizes that it is being pulled with the weight of approximately 8 humans.
Next question: What is softer: Tree bark or human skin ?
Well, all the flying debris does that to tree bark (and this was an actual F5):
I hope the answer to what will happen to two idiots strapping themselves inside an F5 tornado is now clear: They will be mutilated and flayed alive.