94:2 If two courtyards have a door them between, the residents of a courtyard may choose to make one eiruv for themselves. It would then be permitted for the residents of that courtyard to carry in their own courtyard but not to carry utensils that were in the house when Shabbos began from one courtyard to the other. The residents of both courtyards may also choose to make a single eiruv between them, which would enable them even to carry utensils that were in the house when Shabbos began from one courtyard to the other.
If there is just a window between courtyards, they may still make a joint eiruv so long as the window is at least four handbreadths wide, four handbreadths tall, within ten handbreadths of the ground, and it doesn't have any latticework. (A tefach, or handbreadth, is about 3.5 inches.) If the window is any smaller than this, they may not make a joint eiruv. If there is a window between two houses, even if it's more than ten handbreadths from the ground, they can make an eiruv together because houses are considered to fill in the gaps.
94:3 If there are two courtyards, one inside the other, and the residents of the inner courtyard have no means of accessing the street other than passing through the outer courtyard because there is a door between courtyards for that purpose, they may choose to make a joint eiruv. If they do not, but the residents of the inner courtyard made their own eiruv, they may carry in their own courtyard but the residents of the outer courtyard may not. If the residents of the inner courtyard did not make an eiruv but the residents of the outer courtyard did, that outer eiruv does not work at all because the residents of the inner courtyard have rights of access through the outer courtyard, which restricts the residents of the outer courtyard. This is only if the inner residents didn't make an eiruv because if they are forbidden to carry in their own place, they impede those in the outer courtyard, where the inner residents also have rights of access. However, if the inner residents did make their own eiruv, then they are allowed to carry in their own place and their right of access does not impede the outer residents. Similarly, if only one Jew lives in the inner courtyard, so he does not require an eiruv to carry there, he likewise does not impede the residents of the outer courtyard. If two Jews live in the inner courtyard and they don’t make an eiruv, their right of access impedes the outer courtyard, even if only one person lives there.