This is how I made my siamese Rubik's cubes i don't think I am the only one to it this way. But I haven't looked up any plans so I can say this is my take I the Siamese cubes.
I have used :
2x V-cube 3
Sandpaper( various grits )
Step 1: Preparing the Cubes
To be able to work easier we must prevent the cubes from turning. So I taped them in their solved state. After you taped the puzzles then remove the stickers from the pieces that you are going to cut.
Step 2: Cutting the Corners
Using the dremel sanding bit slowly shaved off some of the material from the corners. Goal is creating a straight and even surface for glue to stick.
Step 3: Glueing the Corner Pieces to the Edge Piece to Add Stability (OPTIONAL)
After the cutting process you can apply glue between the cut pieces this part is optional but I think it makes the puzzle more stable
Step 4: Final Assembly
For the final assembly first fill the pieces with your filler of choice( I used hot glue ) and sand it flush. (This step only applies to puzzles that have hollow pieces. )
After the filling process you are ready to smear some superglue and press the puzzles together.
You can put the sticker on top and bottom to finish the project.
Step 5: You Are Done
Your Siamese cubes are ready to play now.
Have fun solving them.