There is a great feature to view a proofread book in two pages view, which is enabled by default by all viewer by Vis Italian Wikisource (MediaWiki:Gadget-vis.js).

It could be the view from MainPage (NS:0) and Index Page ( NS:106). For example if you go to the below two pages..

1. https://it.wikisource.org/wiki/L%27Ossola
2. https://it.wikisource.org/wiki/Indice:L%27Ossola_di_Carlo_Errera.djvu ( corresponding Index page)

Press the CTRL+Alt +V in both page MainPage (NS:0) and Index Page ( NS:106). you view the book page by page.

I have import the same at to Bengali Wikisource
1.https://bn.wikisource.org/wiki/Mediawiki:Gadget-vis.js
2.https://bn.wikisource.org/wiki/Mediawiki:Gadget-common.js
3.https://bn.wikisource.org/wiki/Mediawiki:Gadget-vis.css

And it is working fine from the Index page( NS:106), but not working from Mainpage (NS:0).

Check the different from one book

1. https://bn.wikisource.org/s/65k {MainPage (NS:0)}
2. https://bn.wikisource.org/s/6se {{ Corresponding Index page( NS:106)}

Press the CTRL+Alt +V in both page MainPage (NS:0) and Index Page ( NS:106). It will work from Index page, but not from MainPage.