Comments/Ratings for a Single Item

if a pawn moves 4 on the first move, it yet has to move 6 more spaces to
promote. it gets worse for the pawn in this regard if it only moves 1, 2,
or 3 squares on that first move...

what i'm saying is that, on a board this size you need more mobility from
your pawns... or maybe move the promotion line up a couple or few ranks
from the back line...

when i'm designing a variant i look at how quickly and powerfully the
pawns can promote, and depending on the variant adjust it in terms of FIDE
chess... do i want a game which plays faster or slower wrt to
promotions?... 

in any event this variant seems to make the pawn less important,

and yes the knights on this board are much weaker than they are on an 8x8
board... on a larger board like this the leapers need to have more
range... but that's just my taste...

speaking of the rooks and bishops, i would imagine that the rook becomes
even more powerful than the bishop, because the bishop's maximum move is
11 (12-1) squares diagonally while the rook can theoretically move 15 (16
-1) squares horizontally... it seems to me that boards which aren't
entirely square like this favor rooks even more than bishops....

i think this game would play better on 16x8, but then the bishops become
even weaker in comparison with rooks...

but with 12 ranks, man that is a long ways to go for a promotion! gives it
an old-fashioned feel in that regard...

Does anyone know what the rules say for when both a player's kings are in check simultaneously (e.g. by a discovered check or fork)? Is it "mate"!?

It seems to be an impossible case.

What about to play a game?

Assuming that the laws of check follow FIDE rules (which is a reasonable assumption for a variant player by orthodox chess masters) Checking both Kings simultaneously is quite possible by discovery or fork, it is not automatically mate in the case of the fork, as the checking piece is potentially capturable, but cannot be answered by interposition or King move. Discovered check whereby each King ends up checked by a different piece is checkmate: there is no possible way to answer both checks. A line piece can also check by pinning one King to the other, for example Kings on a1 and c1, b1 and d1 vacant, enemy Rook moves to e1. This can be answered by capturing the Rook or interposing on d1 (not b1).

Note that it might be possible to interpose on a discovered check where ech King is checked by a different piece when the pieces are sliders, and their check rays intersect. E.g.

8/2pkkr2/r7/8/8/4P3/3B4/3RK3 w - - 0 1

1. Bb4++ Rad6!

In fact there is more. The methods for evading each check are limited to capture of the checker, interposition or King withdrawal. So with two checks there are 3x3=9 combinations of these, but only 6 combinations are different. You cannot capture two pieces in one move, or withdraw two Kings in one move, but as shown above you can sometimes block two sliders with one move. The 'inhomogeneous' possibilities are capture+withdrawal, interposition+withdrawal and capture+interposition. Now the latter is not possible with replacement capture, as interposition has to be on an empty square. The only capture in Chess that does that would be e.p. capture, but it turns out that the required preceding double-push can never create the discovered duple check that could be solved this way (because the pawn comes from a different square than its e.p.-capturer will end up on (*)).

Interposition+withdrawal is not possible, as a King cannot be used to interpose. (This is not Chinese Chess, and there are no Cannons...) But capture+withdrawal is perfectly possible:

8/2p2r2/r7/4k3/8/3kP3/3B4/3RK3 w - - 0 1

1 Bc3++ Kxc3!

*) I guess you could do this when one of the checking pieces would be a Moa-rider, as a Moa-rider on d1 can be blocked on both e2 and e3, so after 1. e2-e4 to check Kings on d5 and (discovered) g7 1... dxe3! would save the day.

Double Chess 1

Double Chess 2

My comment on the impossibility that both kings might be checked at the same time was done in relation to DC2, when a player obviously can not put his/her own king in check.

Capablanca and Lasker played Double Chess, an average sort of CV. There are actually several CVs with the word "double" in the name that have two Kings. Maybe Karjakin and Carlsen could be prevailed to play Capablanca's Double Chess next week for world title instead of Simpleminded F.I.D.E. No, naturally because f.i.d.e. founding in 1924 shut out variations such as in castling, and Capablanca Chess on 8x10 in 1920s as well as Double Chess on very large 12x16 here in 1929 were last gasps for Grandmasters critical thinking.

I made a Game Courier preset for Hayward's Double Chess here.