What are Rein's capabilities with others?
The anime does not make this clear; fusing with a unison device simply increases the mage's offensive and defensive powers, and provides other effects, such as setting Signum's sword on fire when she unisoned with Agito.

While originally Hayate and Reinforce Zwei both used the Book of the Night Sky, later Reinforce got her own storage device - the Book of the Azure Sky.

(To provide a more specific answer would be to engage in speculation.)