Before the D10 and D11 appeared on the market, Cat offered a couple of solutions to make their D9s work in pairs. They were offered in both a side by side combination behind a really wide blade for strip mining work and in combination as shown here for pushing scrapers.
Rubber tired scrapers move materials good distances quite efficiently, but they struggle to get loaded. A push during the loading cycle solves the loading issue. Naturally the harder you push the better it works.