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This implies that the opponent cannot capture two men with one en passant move.

Interesting question! I always take 'en passant' by pushing my opponent's Pawn back to the third rank and then capturing it in a normal fashion. The result is the same as if I forced my opponent to retract his two-step Pawn move and then make a different move with the same Pawn. Marseillais Chess rules lead to considerable confusion here. I would be tempted to say that Black may capture the N(c3) in Antoine's example, but may not perform an en passant capture of the P(c4). We may find out that this question has been dealt with before.

No, I'm wrong about en-passant rule. It states:
'A pawn that is moved two squares in one move (half a turn) can be taken
en-passant, even if the pawn moved in the first half of the turn. The
en-passant taking should be done on the first move of the turn. However,
when two pawns can be taken en-passant, this is allowed.'
I have to see the comment that is going to be displayed in a few hours,
because I'm now a bit confused with Antoine's question.
Some clarifications are needed about rare cases, I expect that an
experienced player can give detailed explanations about it.

I've understood the interpretation of en passant capture to mean the
capturing player makes the en passant move 'as soon as legally
available'. For example, if the initial double-step pawn move results in
a discovered check, the check must be dealt with, then, on the next turn,
if the player is not again in check, the pawn may be taken en passant, if
that move is still available. A series of checks would 'push' the en
passant capture along with it. The checks could even be ended by the
double capture move originally suggested. If that move, the en passant
capture ameliorating check, was available, then it would have to be taken
then, or the en passant opportunity would be lost. This could
theoretically occur in a FIDE game, no? Anyway, the en passant capture
would then be available to the other player during his next move, which
would have to be the one-move capture, and not the two-move non-capture.
In which case, the situation described would be a serious blunder, or a
brilliant sacrifice.
This does not hold if Roberto is strictly right, and there is a voluntary
pass by the opponent, for, theoretically, the opponent could have,
instead, made a voluntary en passant capture between the non-capturing
moves.

I don't know much about Marsellais, I have tried it a few times with a novice status, but I have not analyzed rare situations and fine details. I think this move can NOT be done, en passant movement is a Pawn move in which you capture an enemy Pawn moved INMEDIATELY before the en-passant capture. In your example, after the Pawn move you moved other piece in the second part of the turn. I think it is the same if you move the Pawn twice letting it in a position in which en-passant is permissed, you can't take the Pawn because the last move was not a two-steps move in the same PART of the turn, you moved one square in the second part of your turn, and this is the last move to the effects of the game. This is an interesant discussion, and it must be clarified by experienced players. If we are rigurous with the Chess rules that I suppose are translated to Marsellais, if you move a Pawn two squares and it lands in a position in which it can be captured en-passant, and after that you move other piece, this is the last move, so the Pawn can't be captured en-passant, but if you move first the other piece and after that the Pawn, it is vulnerable to en-passant capture, so order can be important to the effects of the application of this rule. Has someone an 'official' response?.

I have completed Zillions implementation of a simplified version of
Marseillais Chess, which I call Simple Marseillais Chess. Implementing the
rules for en passant would have been very tricky, and there seems to be
nothing I can do about getting it to accept checkmate as a goal. So I just
let myself create a new version of the game, then implemented that. The
simplified version is played like Chess with these differences:

1. Each Player normally has two moves per turn.
2. The second move of a turn is allowed only when no Kings are in check.
3. Although a Pawn may move twice in a turn, it may not make the two-step
initial move available in Chess.
4. Pawns may not capture each other by en passant.
5. The object is to capture the enemy King.
6. 3-times repetition is a loss.
7. A player who cannot move must pass.

I've had an additional thought on how to make a Marseillais Chess ZRF more
optimized. Between each player's first and second move, have a dummy
player check whether either King is in check, placing a piece on a
specified location if either King is in check, and clearing the same space
if no King is in check. Then on the second move, each piece just verifies
that this space is empty before moving. This will eliminate a whole lot of
overhead caused by multiple checks of whether any King is in check.

It might also be useful to use two spaces instead of one. Checking both
spaces could be done with an or. Doing this would reduce a bit of
overhead. There could be two dummy players, a white dummy and a black
dummy. Each could first check for a marker indicating that it's side is
in check. If so, it would check whether it's still in check. If it was
empty, it would not have to check whether it's in check. In either case,
it would check whether it's side has placed the other side in check.

Another advantage of this would be the presence of visible check
indicators for each side. Zillions does not normally tell you when you're
in check. This would be a nice side effect of implementing the game in
this way.

I'm thinking the two moves should be of different move-types. The second
move can first check whether the enemy King is in check.

For example, preceed each move of the second move-type with (no-check?).

Link all spaces on board with next direction.

(define no-check? mark a1 (while (or (not-piece? King) (not-enemy?)) next)
(verify not-defended) back)

This searches for the enemy King's position, verifies whether its
position is defended, which means the current player is threatening that
space, then returns to the position of the piece moving.

Zillions of Games comes with a game called 'Double-Move Chess
(Checkmate),' whose description says, 'Checking the opponent is only
allowed on the second move.' To test whether it enforced this rule, I
played both sides. After moving all four center Pawns forward, I captured
the Black King with two moves from the light-squared White Bishop. The
game went like this: e2-e4, d2-d4; d7-d5, e7-d5; B f1-b5, B b5-e8.
Although the rule was stated in the description, the ZRF did not enforce
it.