My guess is that warlord bonuses don't stack with each other. The whole stack gets the bonus of its leader, which is presumably the warlord with the highest bonus. But the bonus of the chief warlord, who's in command of all stacks, is applied to everyone.

So let's say a common jetstone fighter has a bonus of 0, Dora has 2, Webinar has 6, and Ansom has 10. In that case, in a stack with both Dora and Webinar leading. then each soldier has (0+6+10) combat. Dora gets whatever her base is +6+10, while Webinar has his own base +6+10. If Webinar divides the stack, then the part led by Dora will change the 6 term to a 2.