@165thspc, Hi Mike, glad to see you posting! And, yes, as you are also an off-highway truck enthusiast, I’m pleased to elaborate on both of your questions. First, “what is it?” Fair enough:
None other than the mighty Kenworth C500, 8x8 tandem tandem. And what are these used for?
Although this is a Western Star, the chassis fundamentals are similar, used for off road heavy hauling. As to solving the twin steer, that’s pretty easy, as shown below:
No, it’s not that easy, or not for me anyway. In brief, and not paying any attention to the diagram above, I decided to insert a drag link that connects the steering of both axles, like this:
Here are two examples of how I tried to address this. In the top, the axles are linked at steering knuckles and the lower, being the current project, linking at pitman arms. What you can see inside the red circles are: the orange line, which is the ‘drag link’ connecting the two axles, and the yellow lines, tie rods connecting pitman arms to steering knuckles. This does the trick, but isn’t as tight as I’d like. And, as these are twin steer drive axles, and not solid axles, in real life this would all be handled by hydraulics! No need for the mechanical linkage so what to do?.
But this is a model, and, like you, I want the steering wheels coordinated. As I mentioned in a previous post, and as illustrated in the diagram above, there is more nuance to the geometry in the real life solution than what I could resolve.
To my frustration, I haven’t found any typical shop drawings (ie angle of pitman arms, length of tie rods, linkage etc) of these parts assembled in real life (and for the current build, I scratch built the wheel hubs and steering knuckles, so took what I could get), have instead been relying on interpreting photos of trucks shown on auction websites. While helpful, this approach has made me make decisions to get the results I want. If not, I’d still be looking for more info and the build would stop, and I’d likely go on to another project! 
Regarding your 8x8, I wonder if you couldn’t cut a notch between the fuel tank and frame rail to allow a path for a steering linkage? Something to be aware of though is that the wheel coordination between axles is not 1:1. So simply installing a straight bar will not necessarily result in discernable wheel movement, meaning if you turn the front pair of wheels, and haven’t fooled around with setting the correct length/angle coordination between links, steering knuckles and the drag link, you won’t seen much movement in the second pair of wheels.
Something else to consider is tire/wheel diameter, and how close to the chassis, and the extra link they will come, or hit, when being turned - just one more challenge 
Cheers
Nick 