You don't want them placed more than a few inches apart. When I started at my current company, an AP had been set up and they connected an antenna to each jack and put them down seperate hallways. What happens with diversity is only one of the antennas is active at a time, whichever is getting the best signal. This works because the antenna are close and can both always see the signal, it just used the better one. When the antenna are widely seperated as they switch between active states, the users can't connect to both. So whoever is connected to antenna 1 gets kicked off if they can't reach antenna 2. It works great for the people between the antenna's but anyone who can't reach them both gets disconnected randomly.
