Dear Rabbi If one has the custom to light a yartzeit candle for yizkor, can this be fulfilled by the standard 48 hour yom tov candle which one has burning on yom tov to enable lighting for 2nd day yom tov? Or should one have a seperate 48 hour candle (which will not be used for lighting yom tov candles). Or is it better to light a standard 24 hour candle on the second night?

ב"ה Shalom I answered a similar question a while back and I will repeat here again what I wrote then with some minor changes. There is a law in the Shulchan Aruch (514:5) which says not to light any candle which is not needed for Yom tov. The guidelines among the Rabbis what constitutes "a candle which is not needed for Yom tov" were not clearly defined. Therefore the Mishna B'rura ( in Bi'ur Halacha") suggests that when it comes to a Yahrtzeit candle, so as not to enter this uncertainty if the Yahrzeit is considered needed for Yom tov or not, one should light the candle in a room where people eat so the candle will be needed. Even better, says the Mishna B'rura, would be to the light the candle in Shul, because any candle lit in Shul, is considered in honor of the Shul and therefore wouldn't be considered "a candle which is not needed for Yom tov" Even so, the Mishna B'rura writes that if there is no other way, one may light the candle on the second night of Yom tov in any room of the house, since when lighting a candle in honor of a departed one, it is also somewhat a mitzvah. Obviously, when done this way the Yizkor candle would have to be lit from an already lit candle as is the custom on Yom Tov. There are those who say, the yizkor candle may be lit at home even if other possibilities exist. To sum things up until now. Best is to light in Shul. Second best, is to light in a room in the house where people eat , for example next to the Yom tov candles. The third choice is to light them in any room at home. (See Shemirat Shabbat Kehilchata 2nd Hebrew edition, Chapter 13, 6 and Piskei Teshuvot Shulchan Aruch (514:5,14). Your idea of using a 48 hour candle avoids all the above problems. Since you already lit the candle before Yom tov. Since, I have written the previous answer I have seen other Rabbis suggest so as well. However, the candle for the Yahrzeit should not be used as your source for lighting other candles for the second day. (The yizkor candle should be exclusive for Yizkor according to many Rabbis.) (See Mourning in Halacha Chapter 44) . All the best שנה טובה ומבורכת