MB4 255a: Similarity Between Shituf Mavuyos and Eruv Chatzerot (386:1-3)

It is forbidden to carry between one's house and a chatzer or mavui. If items were in a chatzer/mavui at the beginning of Shabbos one can carry between the two. Mid'oraita chatzer and a mavui are reshut hayachid but Chazal required an eruv to enable carrying in them on Shabbos.

Shituf mavuyot is created by having all the chatzerot contributing food and placing it in one of the chatzerot. It doesn't need to be placed in a house

A shituf mavuyos can't be made unless the mavuyot open to each other. When does a locked door ruin the shituf?