How can I get rid of border doubling while stacking two rectangles manually?

I have two rectangles on Canvas, one on top of the other.

The first is located at Y1, the second's height is H and its location is set to Y1 - H. With this configuration I see double-sized border between them. SnapToDevicePixels is true for both r开发者_运维问答ectangles.

How can I rid of this effect?

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Get rid of e.g. the upper rectangle's lower border like this:

BorderThickness="1,1,1,0"

(or -vice-versa, the lower rectangle's upper border)