Sounds good. I don't like variants too much, as they don't realyl help your game as much as a real game and a realistic position, but that sounds like it could be fun for some.
By the way, how did you get two pieces on the same square? That's pretty cool! 
Oh, I just photoshopped that in. You can't do that on the diagrams. :P
Wow, you did a good job, making that yourself! Some skill right there...
G
I don't understand how en passant works.
Say white has a rook on a4, and a pawn on a2. Black has a pawn on b4.
White plays a2-a4; black answers bxa3 e.p.
Does the rook also disappear?
I don't understand how en passant works.
Say white has a rook on a4, and a pawn on a2. Black has a pawn on b4.
White plays a2-a4; black answers bxa3 e.p.
Does the rook also disappear?
Yes. When a capture is made, all pieces on the square are taken.
Interesting idea, why not call it "multidimentional chess" or "subdimentional chess" and let any number of friendly pieces occupy the same square, why limit this only for 2 pieces? At least up to 4 pieces, because it will be enough space within squere to draw those pieces with enough big pictograms. To make game better you should add one more rule: king cant share his square with any other piece and cant move to occupied square as well. Without this rule mates will be impossible in middlegame.
Eh, the two-piece limit was there because OTB it's hard to fit more pieces on a square in a practical manner. Interesting king move suggestion, I'll try to experiment with that and see what works best.
I don't understand how en passant works.
Say white has a rook on a4, and a pawn on a2. Black has a pawn on b4.
White plays a2-a4; black answers bxa3 e.p.
Does the rook also disappear?
Yes. When a capture is made, all pieces on the square are taken.
But I would view it as a capture on a3, not on a4.
Hm, I didn't think of it that way, but it seems that you're right. Yeah, it would be a capture on a3. Sorry for the confusion.
That's a good idea. How would you write notation?
That's a good idea. How would you write notation?
There would be no need for new notation...
Just add the piece name (with one letter) to the notation, if you used to e2-e4 and g1-f3 type of notations. Ng1-f3 insteed, or Nf3.
I mean in double-down chess when you you move a piece on the same square as another piece.
Neat! Very cool.
So this is a variant that a friend and I came up with back in November, and it only occurred to me just today to share it with y'all. It's called "Double-Down Chess" (unless someone has a better name?) and it works like this:
1. Same starting position as normal Chess.
2. Two of a player's pieces can occupy the same square. So, your first move could be 1. Ra2, giving this position:
This allows maneuvers such as 1. Ra2 followed by 2. Ra3 to get the Rook out early.
3. Castling rules still apply normally, as do check/checkmate/promotion.
4. When making any capture, if two enemy pieces occupy the square that the capture is made on, BOTH are captured. This includes en passant captures. This also implies that a White piece and a Black piece can never occupy the same square.
5. A for-instance clarification - given this position:
White may move the Rook onto the square of his pawn to give check. The same idea applies for checkmate. Both pieces on the square can attack and move normally.
This variant produced a few very interesting games, including one in which a win was turned into a draw by the doubling of the pieces on a square. It also removes a weakness of doubled pawns (one blocks the other from advancing? not anymore!). Pieces can also bolster defense in more interesting ways. For instance, a defensively posted Q/N on the same square will cover an enormous amount of squares.
Give it a try and tell me what you think!