Never seen anything like that - no way it came from the factory. Further a dual exhaust on a WD9 makes no sense since the engine has THREE exhaust ports (middle 2 cylinders share the center port as sflem849 and D Slater point out).
I suspect rather that tractor has had a tee fitting installed atop the stock manifold with the tee's run outlets feeding the mufflers.
Incidentally I once ran across a UD9 power unit (same engine as the WD9) in a Bucyrus dragline crane with a straight-piped triple exhaust. It had obviously been "home built" to replace the original 3-leg manifold.