Hide line art seen through upper layer line art?

Question

I'm doing a t-shirt illustration and I need the line-art to overlap, remain transparent (no fills) and not see the line art behind through the front line art. If I have two shapes overlapping, I don't want the illustration behind to be seen through the front illustration. I can't use a fill on the front art, because I need to maintain transparency on all the artwork so that the t-shirt color shows through. In Photoshop, I would simply mask away the bottom layer areas I wanted to disappear. I don't know how to do this in illustrator. I'm working in CS4 and I am a newbie. Any help would be appreciated.
In the example below, the gray circle is in front of the white circle. I want the intersecting area inside the gray circle to be clear so that I do not see the section of the white circle contained within. I can't use a fill because I need both circles to remain clear so the fabric of the t-shirt shows through.

Answer 1

I can't use a fill on the front art, because I need to maintain transparency...
That depends on how the T-shirt is going to be printed. For example, if the design you've shown is going to be screen printed on a gray shirt (as your screenshot implies), you could simply:
Define a dark gray spot color Swatch and apply it to the left circle.
Define a light gray spot color Swatch and apply it to the right circle.
Fill the left circle with the default white swatch.
In screen printing, you need to think in terms of inks, not colors.In a program like Illustrator, white means "no ink." The design would be printed as color separations. There would be no "white" separation unless you defined a white Swatch as a spot color and used it.
JET