Is it possible for a player to be sent off for two yellow card offences before the first yellow card has time to be shown? e.g. where the referee plays advantage after the first offence.
Yes, this is possible. Another example happened in the German Bundesliga, when a player scored the winning goal and immediately got two yellow cards. One for taking his shirt off, one for climbing the fence. Both are mandatory yellow cards (at least in the Bundesliga), i.e. there is no leeway for the referee.