Okay,
You are using 3 point vectors with a stroke shape for the shading effect yes?
The problem is that if you "hide" a part of the stroke (hide the bicep part for a forearm) it shades the "stumpy end" at the elbow with the round cap which is icky and not correct.
Unfortunately that is how shading effects work. The visible shape gets shaded. Nothing you can do to get around that problem unless you use masking. Masking should be very simple since you are using bones to animate the mask, and all arm parts are exactly the same.
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Here is the download of two sample files for v9.2 and v9.5 demonstrating the effect.
shaded-stroke-two-part-arms-using-masking
You can still use the shoulder "patch" idea if you need to but this solution does the trick for the stroke shading.
There are two options depending on which version of ASP you have. v9.5 is easier to key layer order since you can put the arms in a group layer and still control the vectors inside that group from the parent bone layer. Only 1 layer to order. v9.2 will require reordering both the forearm AND the mask layer.
Anime Studio v9.2
Use the exact same 3 point stroke for the forearm, arm mask, bicep. Turn off masking for all the layers accept the forearm. If you are using masking for other layers in the group, you may need to tweak the mask settings depending on the layer ordering.
Depending on how the masking is set for the bone layer, set the masking for the arm mask stroke. The bone masking setting is important on how the forearm is displayed. If you want a nice round end cap at the elbow set masking to "hide all" and the forearm mask to "add but invisible".
Use the hide edge tool to hide the bicep portion of the mask stroke shape (you want the visible part of the mask to SHOW the forearm and have a nice round cap at the elbow overlap).
At this point masking is set and should be working, but you will need to adjust the mask stroke width as there will be a slight "fringe" around the mask because it's exactly the same width as the arm stroke and a bit of anti-aliasing will peak through around the edges. Just increase the stroke width of the mask shape. It doesn't make a difference how much as long as it is enough to hide the fringe. Remember that the bicep "arm" layer is a FULL arm shape and not masked and will always fill in the arm shape behind the masked forearm.
(EDIT: Previously I thought you wouldn't need to worry about the fringe, but no, you do. It would show fringe of the masked bicep over the body if the forearm was above the body. So this step is required.)
Anime Studio 9.5 Nested Layer bone control
If you are using 9.5, you can make layer ordering and set up way easier. Instead of having the forearm and mask layers just loose in the main bone layer and having to move them both in the layer order, put the forearm and mask layers inside another group layer.
In the parent bone layer properties, turn off masking completely on the parent bone layer (not needed any more). Check the "Nested Layer Control" box on the bone tab of the layer settings.
Set the masking of the new "arm group" layer the same as described previously (hide all) and the forearm to "Mask this layer" and the "mask layer" to "add to mask but keep invisible".
Now you can simply change the order of a single arm group, instead of having to manage 2 layers when reordering.
