Navigation switch

Hello my fellow WordPress developers.

I have to design a website which has a navigation on the left side. And two buttons at the top.

Now if a user clicks on one of the two buttons on the top. The buttons have to change position and also the left navigation has to change according to which of the two buttons is clicked. So to make it a little bit more clear. I open a site, I have one button, and below is it’s subnavigation. Next to this button is another button, but it’s navigation is not shown. If a user clicks on the other button, the two buttons switch position, and the subnavigation of the other button is shown.

I have this makes any sense.. ?? The problem is, I have no idea how to do this. An easy solution is to make two similar subdomain websites and let just one button link to one site, and the other button to the other one. However my client wants to have CMS, so he has to be able to add pages and change it’s content. So this means it all has to be in one site. Any ideas of how I can pull this off the easiest?

Or if somebody knows a theme which does this, that would also be excellent, then I could just build from the theme..

Thanks in advance for any ideas or comments