S9 and S9+ differ in certain specs such as display, battery, RAM, storage, camera et al. S9+ seems to have them larger in size. But telephoto camera stands out distinctively.
Taking into consideration that with the passage of time non-expandable RAM becomes a hindrance in performance (such as a smooth UI, app response, multitasking) we might start getting questions regarding performance about S9, but not necessarily for S9 plus. Similarly, people may get into issues with Camera app on S9 plus or something related with it which would be altogether irrelevant for S9 users.
However, there can be issues with both phones which could be tackled in the same fashion, such as, a solution for customizing Bixby button could work the same way for S9+ too. In that case, should we be closing as a duplicate or leave them be?
That said, I still recommend using separate tags for now.