This has nothing to do with Bootcamp. Simply split your hard drive into several partitions with Disk Utility, and run the OS X installer. You will be able to choose whichever partition you want to install onto, and switch between each by pressing ⌥ on startup.
However, please note that this is not compatible with Bootcamp. I.e. you won't be able to have a Windows partition, at least not with Bootcamp (you'd need a solution like rEFIt).
Regarding whether you'd need to buy OS X again or not: as per the OS X EULA:
This License does not allow the Apple Software to exist on more than one computer at a time
…which I'd take for “it may exist several times on the same computer at a time”.
I know that's not what you asked for, so I'll keep it as an aside, but I can't resist saying it: this “partitioning” habit really is a switcher thing. There's no need for that under OS X, especially if you fully encrypt each partition (which I'm curious about, by the way).
The only reason I personally have two partitions is because I am totally dependent on my machine for my daily work, and I keep another OS in case of system corruption. But I aliased my user folder on the emergency partition to the one on the main drive, as an OS without my data is as useless as no computer. This fear of “a trojan” that would kill your system is irrational, and this setup will cost you much more in terms of maintenance (rebooting, keeping two systems updated in parallel…) than the advantages it will bring.
The only way to achieve this with applications that do not have native multi-account support would be to patch the CFBundleIdentifier which is also used by the application to find passwords on keychains. You can change that by editing the Info.plist file in the application bundle. This will break the digital signature of the application, so always work on a copy.
With Reeder being an application from the Mac AppStore this will break the digital signature of the application rendering it non-launchable. So you will need to remove all signs of Contents/_CodeSignature/CodeResources as well. Then this might work unless the application does additional bundle checking that will prevent this. (Which is something I don't know in the case of Reeder.) When that actually works, you will need to launch the application with a secondary-click (right click, two-finger click or ctrl-click) then selecting “Open” from the contextual menu and confirming the subsequent warning from Gatekeeper. (I definitely can not recommend to turn off Gatekeeper completely.)
Of course you will need to do all these steps for each and every copy after each update of Reeder again. (With the risk that Reeder may add additional checks at any time rendering this process not working anymore.)
Other solutions may be to use different RSS clients for your multiple accounts and requesting multi-account support from the developers.
Best Answer
Yes you can.