For animating them?
Really depends on what is acceptable to you.
A 1 to 1 copy can now be done via control rig
You just link it up so that the bones move together with the animated arms.
Having both arms to the same animations looks rather silly in practice though, so this may not be a good solution.
Still, it's probably the Easiest solution, being all done in engine.
Next situation is a hack.
Firstly, you need to add a "root" bone to the secondary arms.
This will be a prerequisite for the ABP way after you are done with the process.
Import the character in again with a “new” skeleton (and update the existing one to include the root for the secondary arms if one did not exist).
Set up re-targeting with the “new” skeleton - here you just feed the name of the arm bones into the re-targeting - ignore the rest. you want to get just the secondary arms to move. (note do not re-target the secondary arms root bone).
re-target the arms with all the animations you need: This produces a set of animations that are tagged to the secondary arms.
Now that you have this set of animations you can re-re-target the new animations to go onto the existing skeleton of the main mesh.
if you made it to here, you have a set of clean animations that involve just the secondary arms, but work with the main (original) skeleton.
IF you are good with C++ you can actually use the name of the currently playing animation to play the same exact Arms version as a montage.
IF you are no good with C++, you will need to modify your ABP to Blend per bone - that’s why we added a root bone to the secondary arms;
Every animation needs to run into a “blend” node, which will blend the re-re-targeted arms animation on top of the original animation.
Compared to control rig, this STILL allows you to play different animations.
Last but not least:
The complex alternative is to animate them in a DCC - blender.
For that you need to first set up a rig - meta human + rigify are a decent start, you can cut the arms of a copy of the metaskeleton, place them in, and because the types are properly set for the bones, it should automagically rig the super-limb for you.
Can take some work to actually do - you may want to check my plugin, as well as mr.mannequin.
Regardless of what you use, the idea behind it is all the same. Get a working rig, animate the key frames for the arms.
In blender you can actually copy poses and apply them to different things, as well as paste opposite. you can get stuff done rather quick that way.