If one does a nirjala fast for Janmastami, highly recommended, then after Sunrise one can take non-grain prasada and then fast from grains till Prabhupada's feast celebration, thus both fasts are observed.
If one has taken non-grain prasada on Janmastami then one can't take prasada again till the Prabhupada feast.
The sastric paran or break fast rules for Janmastami are as follows:
1. After the Janmastami festival (around 12.45) Grains are strictly forbidden.
2. After the astami tithi ending, 3.55 am No grains.
3. After the most auspicious Rohini/Moon conjunction 15:29 Thursday
The more the extent of the fast with devotion the more it attracts the attention of Lord Sri Krishna.
This year Janmastami is more special because of it falling on a Wednesday and the most auspicious moon and rohini conjunction is present at midnight.
Your servant, Ajamila Dasa
Srila Sanatana Goswami states:“If Astami is joined by the Rohini Naksatra on either a Monday or Wednesday then it is known as Jayanti. That day brings great piety.” HBV 15:380
"One who fasts on that day becomes free from the sins of ten million births. He resides in the pure spiritual world." HBV. 15:381“Mother Devaki gave birth to Lord Krishna at the auspicious moment of Jayanti, when at midnight, the Moon was present with the Rohini naksatra in the vrisabha-rasi on the Astami of the fortnight of the waming Moon in the month of Bhadra. Therefore, on that day, one should observe a complete fast and offer worship to Lord Krishna with a controlled mind.” HBV 15: 348-349