In my case, I want to be able to play different animations on either arm of my first person character. I set it up in the AnimGraph to use the slots and bone-blending correctly, but the most recently played montage always cancels the last one, even if it shouldn’t be affecting all the bones.
So, are simultaneous AnimMontages for different bones/slots supported? From what I can tell, I think the answer is no. A bit of clarification would be nice 
There is a tutorial on the Unreal Youtube channel done by Zak which could cover your problem. Take a look at the “Punching” videos of the ThirdPersonPerspective Blueprint tutorials. In one video he shows how to split the skeleton to run animations independently (in his case its upper/lower body) but I think it’s the same issue.
Cheers,
michael
Have a look at this tutorial:
Haven’t tried if you can play different Anim Montages for blending, but you can always play an animation and overlay it with animation montage.
@IMX, that was the tutorial I was following. The problem is, he’s blending ONE animmontage with other normal animations in the graph. I need to be able to call animations for each arm independently; for instance, if “left hand punch” and “right hand punch” were bound to different keys, each punch would need its own montage, but might also have to play at the same time as the other montage if the player presses both keys simultaneously. But I guess this isn’t possible with montages?
@order66 Yeah, I think I’m not going to be able to use montages. I’m going to try a work-around using complicated animgraphs 
Alright, so from my experiments, it looks like there is NO WAY to just say “right now, play an animation at this point in the graph on these bones” without erasing whatever custom animation was playing before, even if it was in a different slot and different bones. You can only have one custom animation playing at any given time. (By “custom” I mean callable as a one-time play from a function, aka a punch or reload animation.) The only ones that can play at the same time on different bones are the looping unchangable anims that are based on player state (such as walk, run, etc.).
I hope I’m wrong about this, because it seems like a serious handicap. Does anybody else have a solution? (I’ve tried using both montages and sequences, they both have the same result, playing one erases the previous montage/sequence.) 
EDIT: Just noticed “Multiple Montage Support” in the roadmap. So it’s definitely not there yet, but it looks like it’s coming
Any movement on this? I just discovered it in 4.15; it is a massive bummer.
Yeah found it possible from long ago here a link Unreal Engine 4.6 Released! search for “Multiple Montage”
Hallelujah! I wish I would have see this post 7 hours ago. Thank you kindly DDemon!
Your tip, which led to these words, made my day:
Animations are now organized into groups. You can play a single montage per group. Want to be able to play another montage at the same time? Simply create a new group for it.
Dang, today was frustrating!.. But I’m so happy now, I’m kinda forgetting all the struggling already