What Badrobot says is correct, but there may be some more information you would want to consider.
If you have a 64 bit Windows 8.1 install, it is probably configured as UEFI. It has to be this way for Secure Boot to work, but to boot to even a Windows 7 DVD, you need to change the bios settings to allow that. There are two separate settings you need to check, and the OEMs set things up differently, so hard to say exactly how your settings would need to be changed.
First you would need to disable the Windows 8 Secure Boot. This is normally controlled through a CSM (compatibility support module) which is enabled to disable Secure Boot. If you boot a Windows 7 DVD and it throws you out about the time the 4 colors come together, Secure Boot is probably still enabled.
Next you need to decide if you want to install Windows 7 in the UEFI or MBR configuration. Since you want to install it on a separate drive, you can do either one. But to boot a Windows 7 DVD in MBR mode, you will have to set the Secure Boot options to boot UEFI and Legacy. The setting usually allows booting UEFI only (which is a possibility for a Windows 7 UEFI install), UEFI and Legacy, or Legacy only. (Using a Flash Drive to install Windows 7 may not allow for UEFI boots. An additional step is necessary for that option to become available)
If you install Windows 7 in Legacy mode, it will put the boot files on the non-GPT drive. If you install in UEFI mode, with the Windows 8 drive present, it will put the boot files in the EFI partition on the Windows 8 drive.
As far as how you pick which OS to boot, you may notice some dual boot situations will require you to reboot your system, after you select the OS, to start the other OS. So using the Boot Device Menu may actually be faster in certain circumstances.
No matter how you install, you need to be familiar with how it looks to boot into a UEFI configuration or MBR. The UEFI configuration is controlled by a Windows Boot Manager, which you would set or pick as the boot device. For an MBR configuration, you would pick the hard drive as the boot device.
You don't really have to understand all of this, but you can use it for reference as you go through the process of installing Windows 7.