You can only have one active/bootable partition. That's where the boot manager goes, which then would handle booting a second (or more) OS.
I didn't know how to explain it properly but that's what I meant i.e. I want to have a dual boot system with two OSes on two partitions. However, it appears my Windows 8 is on a GPT partition and Windows 7 Pro does not get installed on GPT partitions.