SPOILER ALERT for those who haven't finished Aizen's Arc!
Just a guess based on the fact that there were 2 Hougyoku. The fact that there were 2 Hougyoku was disclosed during a flashback of Ichimaru Gin's memory as he was dying.
There were 2 hougyoku, one made by Urahara and another made by Aizen. The latter was then fused into the first as both were incomplete. The fused one is the perfected Hougyoku that was hidden inside Rukia.
Since there was years prior to the story then a reasonable answer would be Aizen was using his own Hougyoku. This is because the other Hougyoku was in Urahara's lab. But then again, since it is Aizen that we are talking about here, another possibility would be that after both was fused, Aizen used Kyouka Suigetsu's Kanzen Saimin (Perfect Hypnosis) on Urahara to make him believe that the Hougyoku was still with him while in actual Aizen took it with him to perform some experiments with the Hollows.
The second theory has a weakness in that why would Aizen return it to Urahara, which suggest that there might be a time limit to Kyouka Suigetsu's Kanzen Saimin. It would be possible to determine which one happened had we know the extend of Aizen's Hougyoku's power. What we currently know is the power of the already fused Hougyoku and there is no way we can know the limit of the two original Hougyoku since Bleach has already ended without it being disclosed. It is safe to say that the original Hougyoku would be weaker compared to the fused Hougyoku, but what abilities each can grant, whether both grant the same abilities or not, well, we wouldn't know.