Having 1/2" as the first layer makes things problematic. But, first, you need to determine if the joists are strong enough for tile. Note, there's a difference between ceramic and natural stone in those requirements...few homes are stiff enough for stone out of the box unless designed for it from the beginning. So, to determine the joist stiffness, you need to know the length of the unsupported run, the joist spacing, the height and thickness, and the type of wood. There are calculators that can help you figure that out, or span tables. One I like is at www.johnbridge.com , enter the forums, and in the blue menu bar, click on 'Deflecto', then enter your numbers to check the suitability of the joists for tiling. Note, the length is NOT the size of the room, it's the whole distance between supports UNDERNEATH the floor.
1/2" is too thin to support tile in between the joists ( the joists must be strong enough along their length as well). But, screwing a second layer to 1/2" is tough, as the screws often tear out, as there's not much 'meat' to anchor in. One way to handle this is to put a second layer that is thick enough for tile on it's own, and just ignor the first layer is there. A second way is to cut out the existing stuff, and put new stuff down on the joists. A third way is to carefully pre-drill the top sheet so you can feel better when you've reached a tight anchor with the bottom sheet before you tear out the threads. If the 1/2" stuff is full of holes or has a 'D' face, it shouldn't be counted on as good for tiling - a 'D' face can have internal voids as well, and shouldn't be used under tile.
once you've got the subfloor set, then you need either a cbu, or a decoupling membrane. With the radiant, I like a membrane better as it will account for the greater expansion and contraction caused by the heat source better than cbu. Personally, I like Ditra from www.schluter.com, but there are others that work well. On an older house, it is not uncommon for the floors to be uneven and not level. Tiling a large format tile like you are considering really begs for a super flat floor, otherwise, lippage will be hard to control, especially for a DIY'er. For that, I'd suggest embedding the heating mats/wires in SLC. This will flatten and level the floor in one step. Then the decoupling layer, then the tile.