Be careful when using altTrigger(x) because if you call it with x = 1, then the trigger is "always" activated and will not cause a change in state until it sees a change from 0 to 1 again. Just like how repeating mouseFire(1) doesn't make you click more. I can see the potential mistake where entering altTrigger(1) makes it work, then entering player.setImageTrigger(1, 1) and observing it not working or the opposite effect of (1, 0) triggering it. Not that you did that or anything, I guess it's just a warning for anyone else reading.
There doesn't appear to be a difference whether altTrigger or setImageTrigger is used, as the server will just call the latter function when it gets the signal. There also isn't client sided image code that requires altTrigger to be used to "replicate" the state change visually and cause the animation, or the akimbo guns wouldn't work either. (Funny to think client sided weapon prediction is just a couple if statements away, though!)
If the animation names are all the same, try swapping out the model for the pistol dts the gun uses and see if the same thing happens. At least then you can narrow it down to some weird model error. An easier thing to do might be replicating the states in the akimbo addon and see where the left hand gun stops working, if anything..
Did you manually call setImageTrigger, ie fcbn(name).player.setImageTrigger(1, 1); ? (and getImageTrigger(1) was 0 before?) In the same environment you tried altTrigger in. Now that would 100% make no sense.