With tata sky, HD+ I can confirm it works. Atleast the video. For sound, I think you'll need to select hdmi/spdif every time you want to watch it from other tv. I'll confirm the audio later once I'm back home.
BUT, why exactly are you looking to do this? First of all, both the TVs will only show the same channel. So you can't watch one channel on the hdmi TV and another on the composite connection. Second, the remote will only work if the set box is in the same room. And third, if the other tv have hdmi too, you don't even need this function from the set top box. Just add an hdmi splitter and connect both TVs via hdmi. Still won't solve the first two problems though.