If your computer is not a Mac, check in BIOS if your mainboard is able to select the boot device sequence.
All recent boards allow to select which drive you can use as first boot device (for example if you have two sata drives connected to the slots SATA0 and SATA1, you can choose if booting from device connected to SATA1).
In this way you can install both operative systems in separate drives.
I suggest to use a single HDD when installing the operative system, just to be sure to avoid a mess, i.e. install first a single HDD and install win 7, then remove that HDD, install the other HDD in the different slot, and install the other operative system.
Once both systems are installed, you can connect both drives and then select from BIOS which one is the first boot device.
Alternatively, you can install one of the two operative systems (preferably the one you'll use lesser) on an external drive connected to an USB port. When you need to use that operative system, simply insert the HDD on the USB port and boot the computer (again, you must set BIOS to boot first from an USB device)
:doh: Michael was faster :bigsmile: