If you want the outer roundrect to be 1px away from the inner one, then the corner radius of the outer rectangle should also be 1px larger than that of the inner one.
Here's a quick diagram that may help you see how it works:
The inner rectangle has a corner radius of r pixels; its corner follows the arc of the inner red circle, which has a radius of r pixels and its center correspondingly r pixels away from the sides of the rectangle.
The outer rectangle has a corner radius of r+1 pixels; its corner follows the arc of the outer red circle, which has a radius of r+1 pixels and its center correspondingly r+1 pixels away from the sides of the rectangle. Since the outer rectangle is also shifted one pixel up and to the right of the inner (and, we may presume, is two pixels wider and taller to compensate), the two circles are concentric, and the distance between them is always one pixel.