Ok, first you should import the 1P_setup.ske skeleton for the arms and both 1P meshes (left and right arm ) . After that you give each of the meshes a 'Skin' modifier and add the bones from the right arm to the right arm mesh . Same for the left arm .
Now start to weigth vertices. Imo this is best done through the 'WeightTable' after setting the advanced parameters to a 'Bone affect limit' of 2.
Set the weigthtable to 'Selected Vertices' and start with the shoulder bone . note that there should be smooth transitions in weigthing so that the vertices on the seam between one bone and the next should be weigthed to both bones gradually. A vertex only affected by one bone mostly has a 1.0 weigthing while a vertex affected by 2 bones with the same strength has a weigthing of 0.5 per bone.
Here's a screen shot of a 3P soldier ( no 1P shots here atm ) with the root bone selected and the weigthing displayed:

Note how the weighting is gradually reduced where other bones take over. This will give you uninterupted joints.
It is of the first importance to have everything reset before starting the weithing process. Also the positions must be very exact on start.
Its a complex process and requires some practise.