seriously?

ok lets demonstrate it.

Grab a 20 lbs dumbbell. Now curl it with your right arm until you can't anymore. Does that mean you can't lift it anymore? No. Just move it to your left arm and curl it until you can't anymore. Does that mean you're done? No. No put it behind your head and lift it with your right tricep until you can't anymore. Then with your left tricep until you can't anymore. Guess what, you're still not done! No lift it forward using your right shoulder until you can't anymore. And I bet you figured this out now, you can also use your left shoulder! You were just able to lift that weight 6 times more than if you just used one muscle to do it. Same thing happens when you spread the load while cycling.