Shifting of current sources
I think it can be more easily explained through the following transformations:


The following is the thought process:


A single current source can be converted to two independent sources
in series. 
The node between two current sources in series can be
    attached to any other node without violating KCL and KVL.


The dashed red line represent the new connection I am making. With little rearrangement you get your circuit.