For downscaling depth, things are a bit more tricky since some of the conventional wisdom used for scaling color images won't necessarily apply. Most people just end up using point filtering to downscale depth, which preserves edges but increases aliasing. To implement that, it really is as easy as you think it is: just use a pixel shader to sample the full-size render target, and output the value to your half-size render target.
My image seems distorted now though. Do I need to change the viewport while rendering in half res ?
edit: Ah yes that was it