So according to an online calculator, the radio horizon at 400 feet above ground level, which is the maximum we can do in the United States, is about 45 miles with a line of sight of 39 miles.
So if you had two drones, 70 miles apart, they could absolutely communicate with each other if they were both at 400 feet elevation.
mind you it would probably be better to have them closer or to have more in between just to get a stronger signal and easier decoding but that just goes to show what's possible