I came up with a ritual that allowed one attunement slot to hold multiple ioun stones. Its in my Bazaars of Port Nyanzaru thread!
Edit to add text:
Congenio's Ritual 2500 sp
Over the course over a tenday, you join the temple's elder in retreat, in an attempt to unlock the mystical energies of Ioun stones. As a prerequisite you must permanently devote one of your attunement slots to Ioun – it can no longer be used to attune any other item than Ioun stones. Then make three DC 20 Intelligence (Arcana) ability checks, each taking 8 hours of time. Each success means one extra Ioun stone can share the attunement slot you devoted to Ioun.