Eiruvin 6:10-7:1

Eiruvin 6:10

If a resident of the outer courtyard neglected to participate in the eiruv, it is permitted to carry in the inner courtyard but not in the outer courtyard. If a resident of the inner courtyard neglected to participate in the eiruv, neither courtyard may carry. If the residents of both courtyards made a joint eiruv and one of them forgot to participate, whether he was a resident of the inner courtyard or the outer courtyard, both courtyards are prohibited. If the courtyards have one resident each, eiruvs are unnecessary.

Eiruvin 7:1

If the wall between courtyards has a window at least four handbreadth by four handbreadths (about a square foot) within ten handbreadths of the ground (about 30”), the courtyards may make individual eiruvs or a joint eiruv. If the window is any smaller than this, or any higher from the ground, they may only make two eiruvs, not one.