The 120-300mm you referred to is presumably the f2.8 Sigma?
A 1.4 TC will lose 1 stop, giving a 420mm f4 combo A 2.0 TC will lose 2 stops, giving a 600mm f5.6 combo Stacking these TCs will lose 3 stops (8 times less light entering the camera) giving a 840mm F8 combo in theory. In practice it is probably worse because forcing the light to pass through all that extra glass will cause some extra attenuation. Stacking 2 TCs is likely to cause so much image degradation that I would not hold out much hope of getting a sharp image. At very long focal lengths, vibration becomes a bigger issue too. Reckon you would be better off using the high MP count of D800 and cropping.
Now the 28-300mm. this lens is f5.6 wide open at the long end. Adding 1.4 TC will lose 1 stop = 420mm f8. the D800 and D3s will still AF but not with all sensors so tracking ability is probably lost if shooting BIF. Adding 2.0 TC will lose 2 stops = 600mm f11. Auto focus capability is lost and manual focus compromised by dim viewfinder image. Stacking the TCs = f16 = no AF and viewfinder image so dark that manual focus virtually impossible.
The 28-300 lens is not as sharp as the 120-300 at 300mm. Not even nearly in the same league, so the image quality losses with added TC or TCs will be worse. Added to focussing difficulties and lower shutter speeds/higher ISO forced by the small aperture, I would not rate my chances of getting even a single keeper with this combo. Try it at home before embarking on your trip