For this I see two Solutions:
- use for each armature an own animation
- put all animations into a single animation
What is now the best to choose?
That depends on in which way you use these animations.
If your arm- or gun-animation is alway the same, then it is probably better to make seperate animations. But if the arm-animation is dependend on the gun (or reversed), then I suggest to put all animations into a single one.
Action ~ combination of animations that are working together
Examples: Idling, Aiming, Shooting, Reloading
Animation ~ A set of keyframes
Examples: IdlingArms, IdlingGun, AmingArms, AminingGun
OPTION: Use for each armature an own animation
Foreach Armature and foreach action you create an animation. Make sure you give your animations meaningful names.
Note: This option is simple but animating can get hard, because for each action you have to make sure, each armature has it's own animation. And you will have many animations to handle.
OPTION: Put all animations into a single animation
Foreach action repeat following process:
- For each armature go into 'Pose Mode'
- Go to the DopeSheet-Editor
- Change Mode from 'Dope Sheet' to 'Action Editor'
- Add a new Animation by clicking '+ New'
- Maybe give your animation a name
- Select all other armatures and select the created animation
Any Keyframe you insert will be in the same animation
In that way you will have the same amount of animations as you have of actions.
Note: In the 'Dope Sheet Summary' you will only see the keyframes from the selected armature.
I hope I could help.