Percentage width in a RelativeLayout

Question

I am working on a form layout for a Login Activity in my Android App. The image below is how I want it to look like:

Answer 1

Interestingly enough, building on the answer from @olefevre, one can not only do 50/50 layouts with "invisible struts", but all sorts of layouts involving powers of two.

For example, here is a layout that cuts the width into four equal parts (actually three, with weights of 1, 1, 2):