Originally Posted by NWIFlyer

I believe not - you would need to cancel before departure time, otherwise you'd be regarded as a no-show. You'd probably get taxes back, but essentially they won't be worth claiming.

Given the cancellation fee on a flexible ticket anyway, I'd think it would be more sensible to just book two cheaper non-flexible tickets and throw one of them away, the cost of which you'll need to balance against arriving at destination a couple of hours or so later.