So from what i am reading here, 2 hard drives is the best option.
Instread of an external... can i just add another internal hard drive ?
Sure.
Have one hard drive with OS X and the other with Morphos ?
Yep.
To choose my boot hard drive i would just have to hold down "C" like when i want to boot of CD ?
No, the "C" key is for CD boot only. To Get the Boot selector screen, you need to press the option key (if you have an Apple keyboard) or the ALT key (if you have a PC keyboard)
Can the second HD be the one with MorphOS ? as OS X is already installed on the factory (first) hard drive and i dont feel like installing it again.
Yes.