Septian, you have a misconception there.
My answer is based on the manga so read at your own discretion
Firstly, Najenda does not have 2 Teigu's. Even though, yes she used to use Pumpkin but that was before she lost both her right eye and arm while trying to run away from Esdeath. It was mentioned, that due to such injuries Najenda only retains 40% of her original ability causing her to lose the power to wield Pumpkin any longer and has passed it on to Mine.
Secondly, she can use Susanoo even though she is weak mainly because of the type of Teigu that it is. An organic/biological type of Teigu. It has lesser stress on the user, hence allowing even the weaken Najenda to use it.