The thing that most people do not visualize when they're imagining how lightning strikes cause damage is this:
The earth in most places is a crummy conductor of electricity. And in the other places, it's an even worse conductor of electricity!
Try to envision the earth in any given area as a 3 dimensional volume of varying resistivity. Then imagine a lighting strike inserting a pulse of, perhaps 100,000 amperes of current into this mass of varying resistance, trying to find its way to "earth ground" (whatever that is).
We know that current times resistance = voltage. As this enormous pulse of current spreads out through the earth from the point where it made contact, voltage gradients appear from any point to any other point within this volume of random resistances.
So let's say we've driven a ground rod in at one building, and another at a second building. And let's imagine lightning striking a tree or whatever anywhere near these two buildings. As the current spreads out from that point of impact, very high voltage gradients suddenly appear between any two points, and certainly between our two ground rods. Suddenly "ground" at one building can be thousands, even tens of thousands of volts different than "ground" at the other building.
It is this voltage DIFFERENCE that causes damage.
And this happens even within a building. Grounds are never perfect. And even if we achieve a low resistance grounding scheme, that doesn't overcome the inductance of the ground wiring. So when we present this ground system with a very fast-rising current waveform, we can still end up with momentary voltage differences from any point to any other because these high currents are prevented from flowing, for a moment, by the inductance of the wiring itself.
The concept of "a good ground" is mostly a fantasy. You can go to heroic lengths to achieve a better ground, but that's about it. And within a large building, or certainly from one building to another, you're just going to have to deal with high voltage differences appearing when lightning strikes nearby. Let alone a direct hit!
Further, high current pulses, like we see with lightning, even in the lightning stroke itself, will radiate an EMP (electromagnetic pulse) that can be quite sizable. And nearby conductors will act as receiving antennas and high voltages can be induced in those nearby conductors as well.
So for connections between buildings, since we know that their "grounds" will often be tens of thousands of volts different from each other, the best policy is to NOT connect the buildings directly with signal wires. Radio links or optical links are the way to go. And even for shorter runs, it's good to have surge protection at both ends of any direct electrical signal connections.
When setting up radio sites on mountaintops, we would drive an array of ground rods into the earth surrounding the radio shack, often 25 or more rods, connect them in a ring using enormous copper cable exothermically welded at every point. You know what was considered a "good" ground resistance for such an array? 25 Ohms! All of that, and 25 Ohms to "earth" was considered good!
Anyhow, this "good ground" was then connected to a heavy (perhaps 1/4" thick) copper plate that serves as the "entrance" for all electrical connections entering the building. This plate is, then, our "ground" reference.
The tower was grounded to that same plate, and all equipment grounds went directly and independently (isolated grounds) to that plate (no daisy-chaining allowed). Every signal or power connection into the building had a surge protector mounted to that plate as the connection passed through a hole in said plate.
This is the concept of a "single point" or "star" ground.
Let's imagine now, lightning striking our radio tower on the top of that mountain. And this happens hundreds of times or more every year for one of these sites because they're always the best point for a discharge in the area. As the lighting strikes the tower, the current pulse goes down the tower and its ground cable to meet our "entrance plate" and the huge ground system for the building and tower. At that moment, let's say this strike is a big one, and has 100,000 Amperes of peak current. What's 100,000 times our 25 Ohms to true "earth ground"? It's 2.5 million Volts! So our "ground" is suddenly at 2.5 Megavolts away from "earth ground"!
So why doesn't that radio station get blown up and be off the air constantly? Because of the single point or "star" ground system. Everything in our radio shack jumps to that same 2.5 Megavolts potential at the same time. So there is no DIFFERENCE in potential between any of our pieces of equipment. So the gear doesn't see any harmful voltage across it or through it. Properly set up, a site like this shrugs off thousands and thousands of direct lightning strikes over its lifetime.
We actually do the same thing when laying out printed circuit boards for analog applications.
The idea is that even if a high current passes through one path to our "single point ground", the voltage drop in that particular ground path will not impose a voltage, with respect to that ground, in any other device's "ground". That's why we cannot abide "daisy chaining" our grounds. To do so is to invite ground-referred noise.
But unless you're going to set up your buildings to each have this sort of lightning-tolerant single-point grounding system, you won't ever find much safety from lightning-induced surges even within a large building. And you'll never avoid high voltage potentials between different buildings.
So you just have to envision the way the current from a lighting strike spreads out in the crummy "ground" of the earth, and realize that you must simply accept that voltage gradients will be induced from point to point on the ground.
This is why they tell you to squat down, making your body into a tight ball, with your feet placed as close together as possible when you feel a lightning strike is imminent and you're caught outdoors. With your feet right against each other, the hope is that the voltage potential between them will be relatively small as the current spreads out in the earth under your feet, so there will be no reason for it to choose passing through you as one of the "preferred" current paths.
Go with the radio or fiber-optic links. You'll never truly "ground" two buildings together in any lightning-proof way.
