How many saddles?
You'll have to ask your GM this one; the Djinni's Blessing is an exception to the rules and doesn't specify. It seems reasonable that you should have a second saddle (or more likely an exotic saddle that seats more than one person) or else the additional rider would suffer bareback riding penalties.
Whose space is this anyway?
Per the Mounted Combat rules, all creatures on the mount would be considered sharing a space.
For simplicity, assume that you share your mount’s space during combat.
There's no reason this would change with 3 targets. You are mounted on your steed, and your ally is mounted on your steed, so you and they are both sharing space with the steed.
Charge! (and other combat stuff)
Since both of you are mounted, you both gain the penalties for the Charge action, and gain the benefits from a Charge if you make an attack at the end of the movement.
If your mount charges, you also take the AC penalty associated with a charge. If you make an attack at the end of the charge, you receive the bonus gained from the charge.
Generally speaking, you won't share benefits from feats, but Ride-By-Attack is a bit of an exception... in part. Strictly by the book, the second rider could make their attack per Mounted Combat rules
If your mount moves more than 5 feet, you can only make a single melee attack.
and the mount could continue its movement because the Asavir allows it. However, because they are not directly benefiting from the Feat, they could be subjected to attack(s) of opportunity.