skink chief + stegadon help + ancient steg + sharp. horns. Since sharp. horns affects the whole model the D3 wounds should apply to the helm too, so it should be possible have a D6+1+D3 hits which inflict D3 wounds each, for an impressing possible value of 30 wounds am i right?
Unfortunately, by the Impact Hits rule: "If a creature is granted two sets of Impact Hits, normally because its troop type and special rules both bestow Impact Hits, use the highest set, rather than the total" So, a stegadon model isn't going to get more than d6+1 impact hits.
Though in this situation the two sets of impact hits are from different sources, if the character is doing one set of impact hits and the mount is doing a second set of impact hits surely you would resolve the stegs hits at S6 with multi wound D3 and the skink's at S4. I don't see why one set of impact hits should supersede the other in this case, in most other respects a character and Monsterous mount are treated as separate entities when striking blows in combat. The question is whether the sharpened horns would affect the chieftain's impact hits as well. Now its fairly easy to say the damn well shouldn't, but if your one of those gamey types that wrings every last advantage out of rules text that wasn't designed for that level of scrutiny then I can see how the question could arise.
I think a Character riding a monster is still considered a single Model. The exception I see in the Ridden Monsters sections is: "Whilst a cavalry model is treated as as single model for the purposes of hitting and wounding, a character an his ridden monster can be attacked separately." So the fact that you can attack the rider or monster separately is an explicit exception to the regular cavalry rule that the rider and mount are considered a single model. That same section says "The whole model is treated as troop type monster..." which seems to imply that Model = Rider + Mount. I'm not sure if there is anything else in Warhammer that had Impact Hits that can ride something else with Impact Hits for precedence so I could be in crazy-town in thinking that. However, if both the character and rider get their Impact Hits, then that would mean they don't count as "the same model" and then the Ridden Monster and Special Rules section would come into play for the Sharpened Horns: "We assume that special rules that apply to a ridden monster do not normally also apply to character<sic> riding it and vice versa" and the Skink wouldn't get the d3 wounds.
And a quick follow up... the Impact Hits rule first refers to "If a CREATURE is granted two sets of Impact Hits" then when describing how to resolve Impact Hits is refers to "The Model" So, probably the best argument for double impact hits would be that the Steg and Skink are different "creatures" even through they are the same "model" so they both get their Impact Hits... but then there are no rules for resolving an impact hit from a "creature" only for resolving an impact hit from a "model". Yay Engligh language usage. I think the intention is that a single model charging can only cause 1 set of impact hits. Just like if a model has 2 sets of Ward Saves it can only use one of them. But the Stegadon's Sharp Horns special rule still isn't going to apply to the Skink rider.