But you can't have 2 kings royal in variants, also, how will the king on the 2nd rank castle?
For the first part, that is just a limitation of the editor which can eventually be removed. For the second, the King being able to castle is not a necessary rule for making a good variant.
this variant already exist and look like this :
Is called "Gay Chess" (we know why).
Also, we can't put two Royal Pieces in a custom position; but i suggest to Admins to make it possible.
With several royal pieces games, there are few new rules (with not capture the king) :
- If a piece check two Royal Pieces of a same player in same time, the checked player can't save both, so is checkmate.
Nd3++#
- If a piece check a king with an other king of the same color beihind, the checked king is pined so cannot be moved, so is checkmate.
Bb6+#
I really hope that several Royal Pieces option can be added to Custom Positions.
if you pin a royal then it goes ,the bishop takes and you are left with one king royal
no, the officials rules are that if one king is checkmated than is game over, not checkmating all the kings.
Otherwise, what's the use of several kings?
so you can take the king
and acually a gm or an fm already made this variant with two kings royal
I just recorded a video of Stockfish playing this variant. It hung a queen with both sides.
Hopefully, the video is viewable. However, it is kind of boring. Also, Stockfish gave up when in check a good amount of the time.
I can't seem to view the video here, so I'll try a different method:
Also, having grandmasters play this position would probably be very interesting.
I really hope that several Royal Pieces option can be added to Custom Positions.
I actually have 2 ideas for new royal piece options, and I think they work best when applied to an otherwise completely normal, or at least mostly normal game.
Survivor Royals) Basically, you don't lose unless all survivor royals are put in checkmate or captured. You do not have to respond to check if you still have at least one survivor royal not in check or checkmate. You are able to put your own survivor royal into check or checkmate as long as you would still have at least one survivor royal not in check or checkmate. Survivor royals can be captured. I think this idea works especially well if you let pawns promote to survivor kings. Survivor pieces are notated by putting a $ before the move.
Pass Royal) Pass royals are able to 'pass the crown' to an adjacent piece as part of their move, making themselves no longer royal, and the piece they pass the crown into a pass royal. The new royal piece must be adjacent to them before the move is made. You cannot pass the crown if you are in check or checkmate, and you cannot pass the crown if it would put you in check or checkmate. You cannot pass the crown to a different players piece, or to a piece that is already royal.
These royal variants make things like gay chess or double royalty chess possible, as well as opening up opportunities for other custom variants that could be really fun to play and lead to complex, interesting games. I think I covered the two simplest and basic variants, but I'd be happy to hear any other royal variant ideas! 
I don't think I've shared this idea with the community yet, but I came up with an idea for a chess variant a while back called Double Royalty Chess. The position looks like this:
Both kings are royal in this variant, but otherwise, it plays like standard chess. The pawns on d3 and e3 can only move one square on their first turn, as well as their subsequent turns. Also, this position breaks Stockfish. Stockfish ended up hanging a queen once in a game from this position. It had actually realized it hung a queen on the next move, and ended up getting checkmated, despite only considering one king royal.