"Simultaneous closing" refers to the process of closing two separate transactions or deals at the same time or on the same day. It often involves buying one property and selling another property on the exact same day. Full definition
Are simultaneous closings on this type of deal possible? (biggerpockets.com)
Doing a double closing has nothing to do with any investor strategy other than it is a closing process that can be used to accomplish simultaneous closings, which is not illegal. (biggerpockets.com)
Another reason is if you are buying from an institution you need to use the double close or simultaneous close because most banks won't allow assignment fees in their deals. (biggerpockets.com)