That's not a benefit, though. Suppose you control the holy cities for both Buddhism and Judaism. You're not going to be able to spread those religions much faster than if you were just spreading one religion because you have to use the same production and the same money, the missionaries are starting from the same place, and you have the same open borders agreements. In other words, you could either build 2 Jewish Missionaries or 1 Jewish and 1 Buddhist, but you can't magically build extra Missionaries by having two religions that you couldn't by having just one. You're just dividing your energies. You don't gain any extra converts and so you don't gain any extra gold. All you do is deny someone else the ability to found that religion, but trying to spread your #2 religion would be at best zero profit and at worst a loss.