Here in your article about installing a Mac OS on PC
How to Install macOS on a PC (Mac Required)
With the right combination of components and effort, you can install macOS on a regular PC.
is the Mac OS a standalone operating system, or is it besides Windows? Like you get to choose what operating system to launch at the beginning?!
Essentially, OS X is installed just the same as Windows is installed on the machine. The way I outlined it in the article, it becomes the sole operating system on the computer.
so how can I make sure to have it as a second OS (besides Win7) on my PC?
In theory you should be able to install it besides Windows.
To run OSX on a none Apple system you are using a Bootloader to emulate the EFI Boot. this Bootloader should enable you to dual-boot too.
Here is a guide for Chameleon Bootloader (http://www.hackint0sh.org/f184/87072.htm)