How to avoid this feathering that two masks make?

Hello. I want to separete a layer , something like this.

I made a star sape and duplicated it, and masked a star(Mask mode is Add) and copied the mask After that I pasted the mask(Mask mode is Add and inverted check box is checked)  to the duplicated star.

But both two stars make this ugly featherd edge.

Please magnify the picture, there is a thin transparent line on the area where two separated stars meet up.

I don't want to feather the edge and make it hard edge(aliasing edge I mean.) and remove this transparent area.

Do you know how to avoid this? Help me please.

Thank you.