Help: how to display two different homepages in alternating order?

Hi
I want to display two different homepages in alternating order, like this:

visitor 1 sees homepage 1
visitor 2 sees homepage 2
visitor 3 sees homepage 1
visitor 4 sees homepage 2
visitor 5 sees homepage 1
visitor 6 sees homepage 2
visitor 7 sees homepage 1
visitor 8 sees homepage 2
etc….

how do you do this?

Thanks!