Masking Question

I’ll attempt to explain this as best as I can.

I’m making a flash intro, and it starts off as a green background with a 1px wide line in the middle. The line extends from the top to the bottom of the movie. I want the line to “grow”, so at frame 1 it’s a 1x1 line, and at frame 25 it’s a 700x1 line.

I know you use a mask, but I’ll need two masks since the line starts as a point in the middle and grows both up and down. So, is it possible to double mask something, using one Mask layer and another layer for the line?

Thanks in advance.