Is there an issue for two sisters both celebrating birth of a baby girl to have joint kiddush?
Mazal tov to the whole family!
I don’t think that there is any issue with doing that. The issue would be because of ayn mearvin simcha bsimcha, however that only applies to making weddings, and not to kiddushim of a daughter, a bris or pidyon haben. You might not want make the kiddush together because of ayin hora, but that would be a personal thing, not a halacha.