For the converted 512s, the tv2 RF out is not active. I've tried.
Side note, I like to think of the 512 (& 612 for that matter) as having 1.5 tuners. You can record 2 shows at once but not do PiP like the 722/625 in solo mode. Also, both need seperators w dpp or 2 lines. If it was an older quad...