I have a similar setup to you, but the PC monitor is upstairs and the HD TV is downstairs, so wouldn't be much point spanning them, LOL.
ZoomPlayer allows you to adjust the aspect ratio of video playback to anything you like, but I think its full-screen (windowless) mode only works on one full monitor or the other. So the best you will be able to do is have a single playback window which stretches over the whole of the two screens, and then fill that window with rescaled video.
DizFan