It won't go full siphon if there is a durso stuck on the other branch, it'll just suck air through the durso. Teeing a return line is fine. Teeing a drain is almost never a good idea especially if you're mixing types or involving a siphon or emergency drain.
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Inconveniencing marine life since 1992
"It is my personal belief that reef aquaria should be thriving communities of biodiversity, representative of their wild counterparts, and not merely collections of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae." (Eric Borneman)
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