Generally, it is recommended to perform the Asthi Visarjan ritual separately for each individual, with a distinct Sankalpa (vow) mentioning their specific name and Gotra. This ensures the prayers and merits are directed appropriately to each soul.
However, if circumstances necessitate (e.g., ashes of both parents collected together, or logistical constraints), a knowledgeable Pandit can guide you on how to perform the immersion during the same trip, possibly with slightly modified procedures or separate Sankalpas within the same session. It is best to consult the performing Pandit about this specific situation beforehand.