Scion of the Ur-Dragon and Torrent of Souls
Determining how continuous effects interact is done using a system of layers (see also: How do Layers work?) 


Copy
Control
Text
Type
Color
Add/Remove
Power


In this case, Torrent and Scion both create continuous effects that affect the P/T and abilities. Layers gives us the order in which to apply them, even if the effects were created in a different order.

Scion's copy ability applies in layer 1. So, there, it becomes a copy of Atarka.
Torrent's haste and +2/+0 are applied in layers 6 and 7. So, "Atarka" gains haste and +2/+0.  

When "Atarka" attacks, it gains another ability in layer 6. It doesn't conflict with anything, so it's just added. So, you'd have an 8/4 Flying Haste Double-Strike after declare attackers.