I've finally started looking into this Bwian (Now that my slinger anim has stopped fighting back ).

The problem your having is just because you're effectively offsetting the mesh from the bones on the x-axis. This can't be done without changing the skeleton to match the offset. I'll give you an example:

Here's the GS slinger. as you can see I've offset the mesh horizontally.



Look what happens when I rotate his right upper arm:



The vertices assigned to the upper arm have to move in accordance with their position in the model. Because the vertices have been moved, they still have to move in relation to the bone they are assigned to, but now the distance is greater so they are rotating in a wider arc. If they didn't, then on the regular models, when a bone is rotated, all the verts assigned to it would move on thier own axis and just rotate in situ.

The reason the horses only appear to be broken at certain points is because the model isn't offset on the y axis. So, for example, I can rotate the offset-slinger's head to look up or down and it is fine, but if I rotate it to look either right or left then it now has to rotate on the offset x arc.



The bad news is that this is unavoidable.

I haven't worked with horses yet so I'll go with what I know: The only way I would be able to get two slingers working together as one model would be to have two human skeletons side by side, with different bone names... possibly with a master root-bone connecting both pelvis.' Then it would need a completely new 'siamese-twin' anim set to go with it.
Then I'd have to find a game engine to use it with because I'd have as much chance of getting it into M2 as platting fog

I apologise in advance but... we may be looking at a one trick pony.
Gah! I can't believe I just said that.

It may be a case of having to settle for the one horse. But hey that's a real achievement!

As a footnote. That slinger is working fine in the game now... except: Because I can't add an extra bone to the hand and get it to convert (and believe me I tried... even with a hex editor which I'm a total dolt with) the only way I could get it to 'sling' properly was to spin the entire right hand . Not an ideal solution but it works and looks ok. Better than no slinger at all.

Your one horse chariot may not be as you originally intended it, but it works and I'm sure will really add something to the game. So as Roy Rogers said to Trigger: "Why the long face?"