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The Nantes–La Rochelle–Bordeaux line is an intercités line; intercités trains have no compulsory reservation. From Rennes to Nantes, you would take a TER (regional train), also without reservation. (Depending on the times, you might end up with a different mixture of TER and intercités, but my point is that this journey does not involve trains requiring a reservation, as long as you don't take a TGV between Rennes and Redon.)
Unless you have some type of Eurail pass that I haven't heard of and can't find any information about, An Eurail pass counts days, not journeys. The pass is valid only on the day you've ticked the box for (there's an exception for night trains which generally count only against the second day). So you can't put two legs of the journey on the same Eurail day if you do them on a separate day.
If you want to avoid ticking two days for this journey, buy one of the tickets separately. The full fare for Rennes–La Rochelle is 43.60€ and La Rochelle–Bordeaux is 28.60€.