Do I need to put something between tool boxes I'm bolting together on a truck?

My guess is that metal does not compress. Cork does so you can tighten the bolt without the need of a lockwasher as the cork compresses.
. Instead of cork I would probably look at mudflaps for big trucks. That is rubber composite so will compress...and also you can cut long strips to be the padding between the metal surfaces. Again you can get away with no lock washers.

Car got its start as a university student project at Swiss Federal Institute involving Technology). Based on a gaggle of students and a professional team leader, the goal was to develop a vehicle that uses as little fuel as probable. By using hydrogen gasoline car, developed in ETH/PSI, as electrical power source, pure water will be the car's only emission. Clean mobility completed hence the educational and energy saving issues with the project.

As the truck drives down the road it will bend and flex a bit when you go over bumps or hit potholes or whatever.

If your toolboxes are mounted directly against each other they will rub and push on each other. This will cause them to squeek as the metal rubs, it could damage the boxes if they push together so hard that they bend the sheet metal. In severe cases the forces could even shear off the fasteners holding them in place (that's unlikely, but possible).

The cork probably serves as a good buffer, the cork is soft enough to flex a little bit as the boxes push against it with the flexing of the truck. It probably doesn't squeak, and it absorbs the forces so there's less stress on the bolts holding the boxes to the truck.

So yes, I would get some cork, or maybe a rubber or foam material (something that will hold up to the weather), and place it between the boxes if you are going to mount them directly against each other.

As an alternative you could simply mount them with enough space in between that they won't hit each other with the bending & flexing of the truck.

He may have done that to minimize the chance of metal squeaking. You could probably accomplish that with rubber or nylon washers.

It will keep them from bumping together, scratching the paint and making noise. The boxes may fit together snugly, but as temperature changes they will expand and contract.

Had a similar set up at one time, water can get stuck in between and start rusting them, so seal those seams and you'll have no issues.