I have a 2011 MBP and I just replaced my optical drive with a hard drive. I wanted to know the easiest and free way to boot up windows 8.1 on one of the hard drives using only the iso and a flash drive.I attempted using bootcamp, but it wouldn't work because I don't have an optical drive anymore.
You should be able to simply boot up off of the Windows USB installer, and install onto the second drive. You may need to temporarily remove the OS X drive before installing, as I have heard windows doesn't like seeing other OS's. You may want to do anyway, just to make absolutely sure you are not harming the OS X install.
Once you have it installed, you will want to install the Bootcamp drivers. You should be able to download them online.