wcap, it's not exactly a complete separation. If you have access to some charts of headphones, you will see that the manufacturers deliberately included some crossover so that 'some' sound from the left passes over to the right, vice versa.
Of course, the blending will not be as natural as it will sound on studio monitor.
Another downside of using headphones is that the soundstage will be constricted. You can't have the sound source right outside your ears and have a soundstage that spans a whole field.
but putting that aside, for the same price, headphones will provide you with sound that's a LOT better. Try looking at some brands like AKG, Beyerdynamics, Ultrasone if you've time.
I'm recommending headphones because (assuming you mix and play with your Macbook) you have to have a good DAC and amp to go along with your studio monitors/headphones. Without a good DAC and amp, studio monitors/headphones are just like big pipes connected to a small pump. You can't bring out much quality. That way you might've been equally fine with just a normal pair of speakers.
Ok too much talking.
