No, reaching another universe through the astral plane is impossible. The multiverse theory claims that there is an infinite number of parallel universes in the multiverse that co-exist with our universe. This theory also states that if you can imagine it, it exists in another universe. Scientists that don't believe in the existence of the astral plane would consider it to be one of those universes. But, in the existence of the astral plane, the astral plane is part of our universe. It is simply a veil that blocks mortal vision from seeing the spirits that roam the universe.