Travelling with Eurostar is great, but what happens when you arrive at the Gare du Nord, dive on to the M4 line and then negotiate your way to the Gare Montparnasse 1et2? The following site takes you through from arrival at Montparnasse to the TGV ticket and departure areas both coming and going:
http://parisbytrain.com/tag/montparnasse-bienvenue/
To find out about any station in France, this may be helpful:
http://www.gares-en-mouvement.com/accueil-fr-1.html
Bon voyage!
