The French Intercites de Nuit offer the option to book Espace Privatif for a supplement. It is available when booking at least one ticket in first class or at least four in second class. For example, a full compartment for two people in first starts at €170 (2*€60 for the tickets, €50 supplement). See also How to book entire compartments on French night trains?.
What are the rules and prices when travelling with 2 people on a 2nd class Interrail? Is it possible to pay for a 1st class upgrade, or would we need to pay the full €170 or upgrade to a 1st class Interail completely?