Based on this we can understand why even if by accepting Shabbos early one can theoretically only accept Shabbos and not render it the next day, there is no reason to bring in Shabbos early and make Kiddush and eat a seudah before the zeman that Yom Kippur will begin. This is because even if one makes early Shabbos it will automatically also be Yom Kippur. When Yom Kippur falls out on Shabbos, there is no separate Shabbos and Yom Kippur; rather it is one kedushah of Shabbos Yom Kippur.
Based on this, the Ohr Samayah ruled that one who had to eat on Yom Kippur for heath reasons does not have to recite Kiddush beforehand. This is because the kedushah of Shabbos Yom Kippur does not have in it a requirement to recite Kiddush and eat. If one is required to eat he may do so with out reciting Kiddush.