Wirt does exactly that, hoisting Dipper up after him and then reaching down to help Mabel. He watches below as his Shadow becomes a mass of snakes, and decides he doesn't like what this potentially says about him as a person - or what Dipper and Mabel's Shadow is trying to say about them.

"O-Okay. They can't get us up here. M-Maybe we can, uh--"

He looks back down the hall the way they came, but before he can form it into a thought there's a sharp gust of wind from below. All of the snake heads of Wirt's Shadow are pointed up at them, and they send some sort of wind attack upward. Wirt immediately ducks down and hugs the beam with both arms so he won't fall off of it.