From all first impressions, this 'spire' is linked directly to the second and third tier AA hotkeys. It in fact seems that the spires eventually become a Tower of Arcanum. Lasting only 1.5 minutes, the third tier would supposedly grant lowest mana cost, highest possible critical spell chance and +150% (theoretical) critical damage.
While I have no concrete evidence that selecting the Second Spire hotkey over the First Spire hotkey provides a 'stacked' or even separate effect, the fact that all of these abilities rely most heavily on adding 3 points into the Fundament of Arcanum AA before you can progress, combined with my own observations, seem to indicate a truly increased % of critical strikes + damage bonus when selecting either button. This leads me to believe that once you've moved into the next spire, you could simply replace the previous spire's hotkey or just rename the one you already have because the effects are almost assuredly cumulative.
What I can say definitively about the spires is that they are just as useful as the Malediction of Havoc and Twincast spells. Mobs that often take 12-15 hits only need 4-6 when you start focusing these abilities. Perfect for kiting and you'll raise some eyebrows in a group or raid, but be careful, you're sure to get the attention of whatever comes under the brunt of these hits if you dont know how to manage the hate that comes with almost nuclear spell casting.
Happy Hunting.
Sonavaa