Setting the Multiview option in the Gospel Parallel Reader appears to do nothing beyond change the wording on the tab.
The Multiple Book View icon shouldn't be showing at all in the Parallel Gospel Reader interactive. The intended behavior is correct on Mac. I've made a case to change this on Windows. Thanks for reporting, MJ!
That button doesn't appear on Mac and (unless you remember something that I don't) I think it's a bug that it appears on Windows. A case to remove that button has been filed. Good catch!
Jinx!