Windows Home Server will take over all hardware when it's installed, so you need to install Windows Home Server first, and the other OS second. In between you rearrange partitions so that there's room for the second OS. As an alternative, you could
install it in a virtual machine, but to install as a vm you will need to meet the minimum requirements of Windows Home Server in your virtualization tool.
Also, consider that Windows Home Server is a server OS, not a desktop OS. It won't be able to back up clients, share media, enable remote access to your data and home computers, etc. when it's not running, i.e. you won't really be able to
test it.
I'm not on the WHS team, I just post a lot. :)