This evening, I searched all sorts of references and research on your question. The underlying requirement for sodium carbonate to revert to sodium bicarbonate is the acquisition of CO2. Sodium carbonate (in a mixture with at least one other compound) can serve as a CO2 "scrubber", removing CO2 from air. This seems to be a limited process in pure sodium carbonate in solution. So my suggestion, other than storing it dry in a well-sealed container, would be to store the liquid in a well sealed container with minimal head space above the liquid (i.e. as little air as possible).
If you feel spunky, you can purchase some wide spectrum Hydrion pH test paper:
www.microessentiallab.com
and set up two containers of the sodium carbonate liquid--one well sealed, and one with free access to ambient air. Use the Hydrion paper to test the pH of each initially, then at intervals of days or weeks. Doing that, you can actually measure the extent to which the pH falls over time for the two storage methods. Measuring a solution of sodium bicarbonate will tell you the lowest pH you can expect.
Bob