I proposed a 2 player variant over on the Spanish forums but haven't added it here. We played once and found it quite fun. It goes like this:
Place a piece of a third color on the first space of each pawn card. These are crocodiles. At the end of each round of turns (after the last player has made their moves) all the crocodiles move one space forward, except if they're already at the third space on their pawn card. In that case they move back to the first space. If a crocodile moves on top of a from, the frog is sent back to the Excuse card.
We found that this added element which neither player can control and which changes the board (a bit like a third player would) also adds a bit of planning.
BTW, I see the problem of having 6 cards to take from in a game with 2 players. Maybe it'd be best to put either 5 or 7 cards so that each player gets to take the last one every other time….