MB4 240a: Requirements for Potentially Merging Two Chatzeiros (372:4-6)

To merge two chatzerot there has to be between them a window or door of 4 square tefachim within 10 tefachim of the ground (at least part of the window/door has to be within 10 tefachim of the ground). What if the two chatzerot have a shituf mavuot? (How to calculate 4 square tefachim for a circular opening).

Carrying between adjacent houses without an eruv chatzerot.

A house making an eruv with the two adjacent chatzerot in a situation where the two courtyards did not perform a shituf mavuot