From your explanation; it 'should' be easy enough.
At present: you have two separate sequences, let's say leftWeapon and rightWeapon, each with it's own Track Keys[Translate,Rotate,Scale]. You want to combine them into a single TmeLine.
Create a New Animation in the Pane, let's call it, "bothWeapons". This sequence is 'empty' at the moment; IE, no keyframe values.
Activate the leftWeapon animation, it's keyframes appear in the KeyFrame Editor.
Select all the keyframes you want to combine from this animation[they should turn BLUE]. Find the Copy Keyframe button in the Editor[topLeft corner], once 'copied' to the clipboard, the Paste button becomes active.
With the leftWeapon keyframes copied to the clipboard, activate the bothWeapons animation, it should have NO keyframes.
Click Paste[next to copy], the keyframes you copied to clipboard in leftWeapon are now pasted into the bothWeapons animation, highlighted green and blinking marquee box. At this point, you can move the keyframes anywhere in the bothWeapons animation TimeLine. Once located where desired within bothWeapons animation, click anywhere in the Editor Track sheet to deactivate the keyframes[moveable] and they should be blue. Part one is done.
Now, go to the rightWeapon animation and repeat the steps above. Copy to clipboard the keyframes you want pasted into bothWeapons animation, then activate bothWeapons animation and Paste those keyframes into this animation. This should create a animation named bothWeapons, and it should contain each individual animation Tracks, it the skeleton is the same, IE, both weapons named correctly in the Scene. I don't think you could have a left and a right Scene and combine the two animations, unless the skeletons are the same.
I hope I understand your issue correctly and explained it easily enough! The basic concept to take away here is: you can copy/paste keyframes from animation to different animation as long as the nodes are the same name in each sequence, translations, etc....