Well, I hope you moved on and aren't still waiting for a reply It would be two hits on the mounted unit. Backstab is a vicious card that often turns the advantage of the mounted unit against it (worse for units like Ogres that normally cannot be hit by bonus strikes at all), as any bonus strikes that count as hits on the unit being rolled against are in turn counted as hits against the unit rolling.
For example, if a foot unit rolls two bonus strikes against a mounted unit, and the controlling player of the mounted unit were to play Backstab, it would only count as one hit against the foot unit, as the mounted unit would ignore the first bonus strike (note, that is not optional as it some times can be with Flag results). The reverse, as your situation was, with the mounted unit attacking a foot unit and rolling two bonus strikes, then Backstab being played by the player controlling the foot unit, would result in two hits to the mounted unit - as normally there would've been two hits on the foot unit.
Hope that helps clear things up. Probably my single favorite card in the game due to the enormous momentum shifts it can cause.
Also, for quicker response to questions such as these, I would suggest using the Board Game Geek forums for BattleLore - found here.