The Chess Variant Forum

This is a simple variant that has been thrown about in my mind for a while. I have no idea if this has been put up elsewhere, but here goes.
In shogi, you can drop captured pieces back onto the board as one of your own, with a few exceptions (nifu, uchifuzume, can't drop pieces where they'll have no legal moves on subsequent turns).
The thing is, you can't drop pieces on occupied squares. But what if - you could drop pieces on squares occupied by your own pieces to make it stronger?
Here's how I think 'stack shogi' would work:

In addition to the drop rules present in shogi, a player may drop a piece onto a square that is occupied by another friendly piece. These pieces will fuse into a compound piece - a stack that moves as its two constituent parts. You cannot drop on a square occupied by a stack.
When promoting, only the piece on top of a stack promotes (which makes a knight on a gold different to a gold on a knight). Placing a piece that is forced to promote on the last rank on top of another piece that is not lifts the forced promotion rules.
When a stack is captured, it splits into its constituent parts, and the player receives both pieces in hand to drop back onto the board. So, capturing a stack (which is technically one piece) gives you two pieces. And if you know your shogi - having the right pieces in hand can turn the tables really quickly.

You cannot drop a piece on the king, and like shogi, you cannot drop a pawn on a friendly piece to give checkmate.

As for applying nifu to this new rule, two options are possible:

• You cannot drop a pawn to form a stack on a file that already has a pawn on it
• You cannot drop a pawn to form a stack on a file that already has a stack that consists of a pawn and another piece.

Here's some interesting pieces that can be formed:

Ninja (忍者/忍): pawn+knight. If the pawn is on top, tokin+knight can make a very scary frontal attack. If the opponent captures it, he will only get a knight and a pawn:

Spoiler (hover to show)

Queen (奔王/奔): rook+bishop. Everyone's favourite FIDE queen. Can it break through traditional shogi defenses? Probably.

Gold Sword (金剣/剣): gold+lance. It can target the opponent's camp from afar and threaten a frontal attack:

Spoiler (hover to show)

Silver Spear (銀槍/槍): silver+lance. Can do the same as the gold sword, but its silver move allows it to move past pawns more easily:

Spoiler (hover to show)

I think it's doable with a regular shogi set, but there is the problem of notation, the fact that a stack with piece A and B is not the same as a stack with piece B and A, and also, a player might forget the piece on the bottom of a stack. Online, you could use respective kanji to represent the pieces, but this won't do for Western players who will find learning the kanji more difficult.
With combinations that are the same regardless of who's on top (e.g. the queen, gold+silver) you only need one set of kanji characters, not two.

What do you guys think?