I know that was effectively true at Nokia in the 2000s, but that was more a sign of corporate dysfunction than a deliberate "battle to the death". (I worked at Nokia in 2010, and the last project I was on turned out to be one that was literally being worked on by four other people across the world, as it turned out.)

I don't think I've ever heard that about Apple, though, at least as standard (even unofficial) operating procedure. There's sometimes competing internal concepts -- the iPhone's operating system probably being the most famous example, with Scott Forstall's team basing theirs on OS X and Tony Fadell's team basing theirs on Pixo OS -- but I don't think they were being kept secret from one another.