Cutting them separately would be the safest. I would bet that crack between them goes all the way to the ground. Notch the first one,
up as high as you are comfortable cutting, then do a bore cut, set your hinge, then cut toward the center crack. It should fall when you get to the center. Put a rope in it before you ever start cutting it case in doesn't fall you could pull it on over.
If you want to cut it as one tree, you can wrap 3 heavy duty ratchet straps around the 2 trunks to hold them together. Put one about 18" above your notch, one right where the trunks come together, and one at least 6 or 8 feet above that. I would wrap each strap around the trunk two times, hook the hooks together, and crank them down as tight as you can. Do your notch and back cut down low. When you cut your notch, you should be notching it on a side where you are notching both trunks, then make your back cut where you are cutting both trunks together. You want to avoid falling it in a direction where you would be putting the notch in one trunk and the back cut in the other trunk. That is a recipe for a disaster.
Words of wisdom here, (learn from my mistakes,) install the ratchet straps so the when the tree is on the ground the ratchet handles will be up and not on the ground under the tree.