How easy or expensive would it be to buy two tickets on the spot?
Speaking to the price part, there are two general rules of thumb about long-distance international rail travel in Europe (both of which apply to Paris-Munich):
- Fares vary and they get more and more expensive the closer you are (like air travel)
- Every ticketed passenger must have a seat, i.e. trains can sell out if they are booked (also like air travel, overbooking of planes aside). This is one of, though not the only, reasons that Eurail passes require reservations for long-distance train routes like this.
- Note: this doesn't always mean you as a passenger can choose your seat; it might be assigned to you automatically
In other words, a last-minute purchase will be expensive. For example, tomorrow's 17:55 direct train from Paris to Munich is €155 per person. The same train on a Saturday three months from now (May 27) is only €85.
That said, it's worth understanding the ticket conditions of your ticket carefully. On SNCF-operated TGV inOui trains, for example, the standard 2nd class ticket condition is:
Billet échangeable (ajustement au tarif en vigueur) et remboursable uniquement avant départ : 19 € de frais dès 6 jours avant départ. Dès 30 min avant départ, billet échangeable 2 fois max (même jour, même trajet) et non remboursable après 1 échange.
This translates to (emphasis mine):
Ticket is changeable (with an adjustment to the fare in effect) and refundable only before departure: €19 fee starting 6 days prior to departure. From 30 minutes prior to departure, the ticket is only changeable twice (for the same day, same trip) and non-refundable after 1 change.
In other words, if you know you're going to miss an SNCF TGV inOui train, you should try to change the ticket -- you will be charged a €19 fee and you will be charged the fare difference, but at least you don't lose the initial value of the ticket. In other words, if you bought a ticket at €85 but now it's €155, you can pay €89 at the last minute, prior to the train's departure, (€155-€85+€19) to change to another train on the same route the same day.