Upvote:1
An email to the airport yielded the following answer:
Dear redacted
Thank you for your enquiry.
We are glad to inform you that you may use any preferred security line (in any terminal) to enter the security area at Schoenefeld Airport.
Further information you may acquire via the following link: http://www.berlin-airport.de/en/travellers-sxf/at-the-airport/security-checks/people-and-hand-luggage/index.php
In other words, passengers who check-in at SXF and find the security queues in their terminal long, are fine to use the security queues elsewhere. There's a live indicator on the airport website.