Quick question about Subdiagrams

I have a Use Case scenario that I’m detailing with a Sequence. Basically, I have another Sequence diagram that I want to add as a Subdiagram to two different Lifelines (in totally separate Sequence files). Basically these separate sequences both call a function that I only want to have one sequence of, and have it as a subdiagram of both Lifelines that called it, but when I try to do this it removes it from the first one. If I try to re-add it, it removes it from the 2nd one. I can add it as a diagram “Reference” but that’s not the best solution. Thoughts? I’m using SDE-EC Standard.

Nobody has any suggestions? I find that hard to believe.