I am about to start a 2 piece mould and would like to know if there is a maximum time between applying the Tooling Gelcoat & Coupling Coat?

It's not as much of a problem to not do the main reinforcement within the 'window' of the skin coat. Ideally you would laminate it within 24 hours, however if that can't be done then you can simply scuff up the skin coat layer with a bit of 60 grit sand paper to give the tooling resin some mechanical adhesion, and clean it with acetone before laminating.