The Megatwinx is a very simple modification. I just took one piece out of one megaminx, and then glued the other one!
(I had to take off the tiles from that piece though, as when I glued it the first time, one of them came off!)

It is the mod that allows excellent freedom of movement, but when it stands on both megamince it actually stand on two edges not faces because of the angle difference.

The only pieces that cannot change color are the edges that are adjacent to the place where the corners are connected. But the rest can move freely! The corner of one megaminx fits perfectly inside the empty space left by the corner piece of the other megaminx. And of course, as metioned, it will be better to take off the tiles and fill the emtpy space inside the piece (it has got to be solid!) before applying some strong glue.

Now, by connecting three pieces (two corners and an edge) we will have the "Megatrinx", it will be able to stand on the two faces, but there will be partial movement. In addition, four pieces will be needed to be cut in half and then glued.

Finally, by conneting two entire faces (i.e. five corners, five edges, and a centre in common) we will have the "Megapentix" and movement will become very limited.