So I have been using a western digital caviar green for awhile now that is 2 tb's....these are pretty slow drives, and I believe it is dying on me, so I bought a copy of windows 7, an SSD, and some 2tb 7200rpm HDD to sort of replace it, although I would like to move everything that I really want to the new HDD, but I can't imagine having 2 instances of windows going at the same time is a very good idea....
As long as your system knows which hard drive (or SSD) to boot from to run Windows, you needn't worry about having Windows installed on another physically separate drive. There is no way that both Windows installations will run together.
Just make sure that when you install Windows 7, you only have one drive connected (the one you want to install Windows 7 on). After it's installed, make sure it starts and runs okay, set up your user account etc.
Then power down and connect the old drive that's got your old Windows installation on it. Start up your PC and it should still boot into Windows 7, then you can transfer any files you want to keep from the old drive.
Install the SSD. Go into your BIOS and choose the CD-ROM as the first boot device so you can install windows. Also choose the SSD as the first hard drive to boot, and choose hard drive as the second device to boot after your CD-ROM.
When installing windows make sure to go custom and install to the SSD. Once installed it should boot from the SSD windows. Once in just go partition and format your new 2TB. Transfer your data from the old 2TD to the new one then you can repartition and format the old 2TB.