It's not only who knocked them off, but how. There's no indication they were stabbed and you couldn't garrot both of them. Someone would have had to murder them near the drainage ditch or otherwise you would have to drag them there. To kill them with a pistol would have required a silencer since a minimum of two shots from a pistol othrwise would have been heard all over the compound and quite possibly exposed the killer.
Fredo had access to men who could carry out this task you speak of. And of course with a Hyman Roth sponsored hit you know there's plenty of $$$.
And let us not forget that the bodies were originally lying outside
Fredo's window.
