On Shabbat Mevarchim, Birkas Hachodesh, a prayer announcing when the Rosh Chodesh will be, is said after Kriat Hatorah before Mussaf, containing the following line (with some variations depending on the particular nusach):
ראשׁ חדֶשׁ פלוני יִהְיֶה בְּיום פלוני הַבָּא עָלֵינוּ וְעַל כָּל יִשרָאֵל לְטובָה
When Rosh Chodesh will be for two days, both are mentioned:
ראשׁ חדֶשׁ פלוני יִהְיֶה בְּיום פלוני וּבְּיום פלוני הַבָּא(ים) עָלֵינוּ וְעַל כָּל יִשרָאֵל לְטובָה
This past Shabbat, being Shabbat Mevarchim when Rosh Chodesh will be on the following Shabbat and Sunday, a member of the congregation reproved the Chazzan, who used the above nusach, claiming that when Rosh Chodesh will be on the following Shabbat and Sunday the nusach is changed slightly, and should be the following:
ראשׁ חדֶשׁ פלוני יִהְיֶה בְּיום פלוני ולמחרתו בְּיום פלוני הַבָּא(ים) עָלֵינוּ וְעַל כָּל יִשרָאֵל לְטובָה
I had never heard of this before, but are there any sources which discuss the proper nusach for such a situation?