"5 If one inadvertently committed one transgression, but there are two persons involved and he is unaware of the one with whom he committed the transgression, he is liable.
What is implied? There were two women in the niddah state. Inadvertently, he was intimate with one of them, but he did not know with whom. He had two sisters. Inadvertently, he was intimate with one of them, but he did not know with whom. In these instances, he is liable for a sin-offering. To what can this be compared? To two lamps that are burning and one extinguished one of them on the Sabbath without knowing which one he extinguished or two pots of forbidden fat from which he ate one and yet, he did not know the identity of the one from which he ate. In these instances, he is liable. Similar laws apply in all analogous situations.
ה שָׁגַג בְּשֵׁם אֶחָד וְהֵם שְׁנֵי גּוּפִין חַיָּב. כֵּיצַד. שְׁתֵּי נִדּוֹת וְשָׁגַג בְּאַחַת מֵהֶן וְאֵין יָדוּעַ אֵי זוֹ הִיא. שְׁתֵּי אֲחָיוֹת וְשָׁגַג בְּאַחַת מֵהֶן וְאֵין יָדוּעַ אֵי זוֹ הִיא חַיָּב חַטָּאת שֶׁהֲרֵי יָדַע עַצְמוֹ שֶׁל חֵטְא. לְמָה זֶה דּוֹמֶה לִשְׁתֵּי נֵרוֹת דּוֹלְקוֹת שֶׁכָּבָה אַחַת מֵהֶן וְאֵין יָדוּעַ אֵי זוֹ נֵר הִיא. אוֹ לִשְׁנֵי תַּמְחוּיִין שֶׁל חֵלֶב שֶׁאָכַל מֵאֶחָד מֵהֶן וְאֵין יָדוּעַ מֵאֵי זֶה תַּמְחוּי מֵהֶן אָכַל שֶׁהוּא חַיָּב. וְכֵן כָּל כַּיּוֹצֵא בָּזֶה"
Why does the Rambam need to compare this case to anything, and specifically bring the two examples of two candles, or two plates of fat?
If he didn't bring any example, what would we have thought?
Similarly, if he only brought one of the two, what would we have thought?