[SOLVED] Enabling/disabling sidebar

Basically, I need an option for my users to disable the sidebar, while looking at the posts. I can do that now, but that would disable it for everyone, I'd like to offer that feature to only those who need it.

Because some people, with low 4:3 resolution, will see my images (that are 800px wide) in the sidebar.

I tried installing two Atahualpa themes and a theme switcher. Then I edited one theme's settings that it would not show a sidebar in posts, but when I switched back, I noticed that those settings "carried over," which means it either showed the sidebar for both or for neither of them.

Is there any way I could solve this problem?

Alternatively, could I change the sidebar, so it would be up or down instead? And possibly, so I could only change that in one theme, so people could use that Atahualpa theme, which has the sidebar on the side.