The mount hippo ability does NOT make your unit mount another unit.
It's a morph ability. When you mount your archer to the hippo they do not mount, they are deleted from the map and a hippogryph archer is placed in their place.
(or maybe one of them morphs to hippo archer and the other is deleted, but most probably the first)
And for the question how to make a footman mount a kodo to result in a third unit, well..
Open your ability editor, if I remember right this skill consists of two parts: 1.) the archer skill 2.) the hyppogryph skill. Look up both, in the parner unit field switch the archer to a footman the hyppo for a kodo and in the resulting unit field switch the HG Archer to your desired unit.
Tadamm.
Oh, don't forget to make it independent from the hippogryph training research.