SM64: The Parallel Universes is a ROM hack created by spk for the Mario Jams Duality Competition, in which it placed 1st.
This hack contains one course, but with two different versions of it the player can quickly flip between using the L button. There are some key differences between each universe, including many platforms, obstacles, Stars, enemies and coin formations. All of the Stars require Mario to flip between the two universes at some point in order to reach the Star, which may arguably put this hack into the puzzle-solving genre.
In the hub area, there is a Star door requiring seven Stars to enter.=; doing so will take the player to the endscreen.
Warp between two parallel universes to collect 7 stars and reclaim a stolen treasure! Or something like that! Placed 1st in the Mario Jams Duality themed competition.
Via RHDC
Stars[]
This hack contains one course called Twin Worlds, with the level existing in two universes Mario can flip between using the L button. One is referred to as the green universe, which has a more natural appearance
Act 1: The Tower Atop The World[]
Climb up to the top of the course, flipping between both worlds to make use of the platforms present in only one universe. The Star at the top is only present in the pink universe.
Act 2: Dimension Hoppin' Platforms[]
Navigate up to the clifftop one step lower that the clifftop where the tower is located. Stretching out across the horizon is a series of small platforms; some are semi-transparent and some are solid.
Both of the universes have opposing platforms, meaning that Mario needs to flip between universes when jumping between the platforms. The Star on the last platform is only present in the green universe.
Act 3: Under The Stump[]
Halfway up the course in the pink universe is a tree stump surrounded by a coin formation. Stand on the stop and switch universes- this will cause Mario to fall the short distance to the ground, as the tree stump is not present in the green universe. Then switch back to the pink universe, and Mario will trigger a warp point under the tree stump.
The area Mario is warped to is simply the inside of a tree in the pink universe, but flipping back to the green universe will take Mario to a steel facility. The Star is located in the green universe only, and in order to reach it, Mario needs to wallkick up to it. The catch is that the two walls are both in different universes, so Mario needs to flip between each universe between every wallkick.
Act 4: The Caged Cave Star[]
Head on up the course to the cave entrance, which is present in both universes, and enter. Follow the path along the left wall to reach the Star in back left corner of the room.
This will require Mario to flip between the universes to lava-bounce and dodge the cage walls, as well as to perform wallkicks similar to the ones required to retrieve Act 3's Star.
Act 5: Paisano Pizza Delivery[]
In the green universe head over to the left of Mario's starting point and pick up the Koopa's box. While holding this, Mario can carry it with him when he flips dimensions. Then, Mario can either perform a tricky jump in the green universe to the left, or Mario can take the route to the right, which is much longer but easier.
There is a Koopa present in the pink universe who is lamenting about pizza not having been invented yet- all Mario needs to do is walk close to him while carrying the Koopa's box (of pizza) and the Star will spawn.
Act 6: Red Coins In the Sand[]
Return to the cave containing Act 4's Star. All of the red coins and the Star's spawn point are located in the sandy version of the room, in the green universe. Mario needs to use the platforms present in both universes to collect all of the red coins. Do keep in mind that all of the sand in the green universe is lethal quicksand, and any discoloured patches are only there to indicate where a platform will be in the pink universe, not to indicate a safe patch to stand on.
Coin Star[]
The blue coin switch is at the top of the pink universe, at the base of the tower. It will spawn three blue coins nearby, and three blue coins in a similar formation in the green universe, for a total of 30 coins. There are also a lot of coin formations in this course spread out over both of the universes. In the main area for this course (both universes), there is a total of 133 coins. There are at least a further 39 coins in the sub-area containing the red coins.