Spellbane says it can suppress other castings of itself. What happens if there's an area affected by Spellbane A, which cancels Spellbane and Magic Missile, and Spellbane B, which suppresses Spellbane and Fireball? What spells can't be cast in this area?
It is not normally possible that the two effects would begin to occupy the same region of space simultaneously, and this turns out to be important. Aroden's Spellbane is an emanation, which means "[the spell] functions like a burst spell, except that the effect continues to radiate from the point of origin for the duration of the spell". If, for example, line of effect is blocked for the Spellbane for some reason (perhaps a wall is in the way), the Spellbane does not function in the areas that are blocked off. The same principle applies to generating Aroden's Spellbane through a blocking Aroden's Spellbane: Antimagic Field does (or should) block line of effect, so the new Spellbane effect treats the area of the old as a region it does not have line of effect to. This means that until the point of origin for the emanation is moved into the field, the defending Aroden's Spellbane isn't merely suppressing the newer spell, it completely prevents the areas from overlapping at all. Once the point of origin is moved into the field, however, the defending Spellbane does suppress the newer one, causing it to effect no area, not even the area it would effect that does not overlap the older spell. If the newer spell stopped just short of moving its point of origin into the defending Spellbane, and then the older Spellbane was moved towards the newer one, the newer Spellbane would resist the advance just like the older one did. The important thing is which one currently effects a region and which one is trying to gain control of that region.
While this was derived from the RAW for emanations, it is worth noting that it also causes Aroden's Spellbane in this case to conform to the rules for anti-creature abjuration effects: "If an abjuration creates a barrier that keeps certain types of creatures at bay, that barrier cannot be used to push away those creatures. If you force the barrier against such a creature, you feel a discernible pressure against the barrier. If you continue to apply pressure, you end the spell". Aroden's Spellbane is an abjuration effect, albeit not an anti-creature one, so this kind of limitation popping up in edge-case evaluation is very fitting.
Weird stuff in weird edge cases
While it's not normally possible for two 9th-level spells to go off at the exact same time, it is possible in theory. The most likely method of accomplishing this is to have two castings of Contingent Scroll on scrolls of the two mentioned Aroden's Spellbanes used to create two contingent Aroden's Spellbanes attuned to the same trigger. When the trigger occurs, both scrolls go off simultaneously. In any case, I would use the rules from Waving Hands as interpreted here to evaluate the simultaneous resolution, treating Aroden's Spellbane as a Waving Hands Counterspell/Dispel Magic in terms of simultaneous resolution with other spells.
This results in both spells fizzling as if they were cast within an existing Aroden's Spellbane banning them (if their points of origin lie within eachother's area) or a weird zone in which both Spellbane's both cancel out and continuously reassert themselves, resulting in the effective banning of most spells banned by either Spellbane in the effected zone (including additional Spellbanes), since any such spell cast into the area will be treated as occurring simultaneously with a Spellbane designed to stop it. This zone can shrink in size (e.g. when the origin points are moved apart from each other), but not normally grow in size (even if the origin points are moved closer together) as the rules for relative movement follow the same rules as outlined above.
What they should be doing
Unlike a true Antimagic Field, Aroden's Spellbane is not RAW resistant to Mage's Disjunction. Choosing Disjunction as a disallowed spell only prevents disjunction effects from being able to be cast (or otherwise produced) inside the radius of the emanation. A caster outside that radius can hit your Spellbane with a carefully targeted Disjunction and, if successful, continue on to disjoin any effects within its radius that would have been protected by the dispelled Spellbane, just as a Fireball (or any other burst spell) can continue on after destroying intervening barriers. Other dispelling effects can also work, but have various drawbacks.
My understanding, which unfortunately I can't draw any more RAW support for than the spell description, is that no spells would be banned in the overlapping area, as each spellbane cancels the other, making a normal-magic zone between the banned-spell zones. A third caster could cast magic missile, fireball or even spellbane in the overlapping area. If spellbane C didn't also cancel spellbane, however, it wouldn't function or have any effect in A's nor B's non-overlapped areas.
Note that with your example spells, fireball casters still need to be careful, as not only will the fireball not damage anyone in B's non-overlapped area, but if the line of effect from the caster to the targeted area passes through even a corner of a square in B's non-overlapped area, the spell can fail. Magic missile casters, on the other hand, are free to fire on anyone outside A's non-overlapped area, assuming A doesn't entirely fill the corridor or wherever they're fighting, as magic missile will go around and strike from B's side of the Venn diagram if it needs to.