Terms do not directly address your scenario:
Elsewhere I see this FAQ, but it doesn't explain how 24 hour cancellations are actually processed:
If anyone wants to try booking a Delta flight with two $50 eCerts and a credit card, then cancelling within 24 hours, and letting me know if the eCerts get refunded as one $100 eCert or back to the same $50 eCerts, I would be very grateful
I really don't fly revenue fares on Delta enough to use a bunch of $50 eCerts- I would pick United instead for the TravelBank. (but prefer Delta to United if the eCerts can be combined using this method)