Well as explained in the anime the voids are linked to Mana, so having his memory of Mana being locked away, he couldn't use the voids at full potential, so regaining his memories when he and Gai went to defeat Mana gave him the power to properly use the void genome and extract people's void and give it to them.

Plus combining voids was done in the first half aswell when Shu combined Inori's and Keido Kenji's void to make that gun that destroyed the satelite lasers.