for a perfect sweetheart cake with ROSE’S heavenly CAKES: Torta de las Tres Leches (page 211), use this logic:
1- the recipe calls for a 9×3″ round pan.
2- the sweetheart pan is 9×2″ and has the same volume as a 9×2″ round pan.
3- a 6×2″ round is slightly LESS than 1/2 of a 9×2″ round pan.
4- for the remainder cake batter after filling the 9×2″ pan (1/2 to 2/3 full), use a 6×3″ round pan.
in other words, i baked the 9×3″ recipe using a 9×2″ plus a 6×3″
note: this math works only for biscuit de savoie as far as i know.