- unfold rig, make head_2 and top selectable ( cursor icon next to eye icon);
- select head_2, in 3d view press Shift D, Esc, do the same with top. Join them with Ctrl J;
- select the edges where the head is mean to be connected with the body, Remove doubles with Merge distance set to 0,0001;
- select the hair, add Data transfer modifier, choose the joined mesh as Source object, set the rest like in the pic, apply.
There may be stretching which you can deal with the following way:
- switch to uv_1 map, switch to Edit mode ( TAB), enable sync, switch to Face select;
- press B and select these stretch faces, in 3d view press NUM3, disable Limit selection to visible, press B, hold Shift, and deselect the front;
- press Shift H to isolate the selection, select vertical row of vertices on one side, press W and choose Align X to straighten, press G X and move them to the other side, press Alt H to reveal everything.