The predicate carries the primary substantial-equivalence argument; a reference device only supports one specific characteristic.
You can only have one true predicate story (even if it's a split predicate across two devices) — that's what the SE argument is built on. A reference device is supplementary: it's cited to support, say, a specific material or a specific performance claim, without being the device your intended use is compared to. Treating a reference device as if it were a second predicate is a common way submissions get muddled, and it's exactly the kind of thing that makes a reviewer's job harder.
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