It should be possible, but it probably wouldn't be stable for very long. You'd need a flat area where Entwash is forced to expand out, but you'd also need a resistant ridge along Anduin to keep Entwash from reverting to a single channel. The Okavango delta in Africa is an example where a single river forms a delta inland and then reverts to a single river lower on ( https://en.wikipedia.org/wiki/Okavango_Delta also lists a couple of other places where this sort of thing happens).