Asavir prestige class grant Djinni’s Blessing (Su) : “The mount … carry a second rider without penalties”, how rule it?

Regarding the asavir prestige class :

And at the read of the Djinni’s Blessing (Su) : When an asavir reaches 4th level, her mount receives the blessing of the djinn, making it swift and tireless. The mount’s speed increases by 10 feet, it doesn’t take movement penalties from wearing barding, and it can carry a second rider without penalties. When the asavir reaches 8th level, her mount’s speed increases by an additional 10 feet.

I’m asking how rule it ? Does both are on the same Saddle ? or two distinct Saddle are need ? In the particular case of a mounted charge, the two riders and the mount declare charge (like for one rider and mount) ? in the case of ride-by-attack feat, does the two riders need it to performed it ?

Does the two rider (considering two M riders on a L mount) are considering adjacent or sharing the same space ? in the second case with or without the malus due to it ?

Thanks for your responses.