לרפואת חיים מנחם בן פרימט רויזא בתוך שח"י
A huge "ding-a-rye" [argument] in the achronim - Does a choleh make kiddush when he eats on Yom Kippur that falls out on Shabbos. The Ohr Sameach says that he does not because this Shabbos is not just Shabbos that happens to be Yom Kippur but a Yom-Kippur-dike-Shabbos. The Shabbos is suffused with the kedusha of Yom Kippur so even Shabbos tells you not to make kiddush. [The Kedusha of Yom Kippur is actually defined by the Rambam as a "Kedusha of Shvisa" so it is not at all absurd to mingle together the kedushos of these seemingly different days]. R' Akiva Eiger holds that one DOES make kiddush. For a healthy person, Yom Kippur stands and says "No Kiddush" and we respectfully obey. But for a choleh, Yom Kippur graciously steps aside and says "Come on in Shabbos, I can't assert my authority and prominence, for this, nebach, choleh." So kiddush should be made like on every other Shabbos.
Which brings us to another question: Why don't we healthy people accept upon ourselves Shabbos, say, a half hour before sunset, make kiddush and have some mezonos, then accept upon ourselves kedushas Yom Kippur? This way we fulfill the mitzva of Kiddush and in no way compromise the kedusha of Yom Kippur. We will fast the full 24-plus hours and still fulfill our Shabbos obligation.
Neichzee Anon ["lets see" in Aramaic, even though it sounds like an organization for addicts:-)]: According to the Ohr Sameach this would not at all be neccessary, for this Shabbos is a Yom Kippur-dike-Shabbos, so by accepting upon oneself kedushas Shabbos he is in effect accepting upon himself a Shabbos that has no kiddush obligation. It could even be that it is not POSSIBLE to accept upon oneself Shabbos without Yom Kippur or vice versa because they are intertwined and the acceptance of Shabbos contains within it an acceptance of Yom Kippur. According to R' Akiva Eiger who holds that the two days are viewed independently of each other, perhaps one could accept one without the other. It is noteworthy that no posek ever ruled that one should accept Shabbos early and make kiddush.
There is a very famous machlokes beteen the Taz and the Maharshal. The Maharshal says that when one accepts upon himself the obligation of Yom Tov Shmini Atzeres while it is still the seventh day of Succos - the obligation to sit in the succah still applies [therefore he shouldn't make kiddush because he would have to make a "לישב בסוכה" and it would be silly to make a לישב on the kiddush of Shmini Atzeres]. The Taz differs and says that by accepting kedushas yom tov, the seventh day is over for all intents and purposes and the eighth day has begun.
Their argument is whether the act of accepting tosefes Yom Tov creates for this person a new day [Taz] or merely adds kedusha to the already existing day [Maharshal]. Perhaps we could apply their argument to the question of whether one can accept kedushas Shabbos without kedushas Yom Kippur or the converse. According to the Taz who contends that the tosephes ushers in a new day this would be impossible whereas the Maharshal would say that it is possible for accepting Shabbos does NOT make it a new day which is also Yom Kippur but merely adds the kedusha of Shabbos to Friday.
עפ"י האחרונים ועי' בס' רץ כצבי שבת סי' י"ב, ובקובץ הערות חב"ד סוכות תשע"ד, שיעורי מבשר טוב על ברכת המצות ח"ב סי' י"א, פחד יצחק יו"כ מאמר א ועוד