Two unconditioned dhammas? In A history of Indian Buddhism: From Sakyamuni to Early Mahayana, Professor Hirakawa states:


  Both nirvana and space were considered to be unconditioned dharmas. This fundamental classification of dharmas into conditioned and unconditioned is found in the Agamas.


In what passage from the agamas (and/or pali?) we read space to be unconditioned?
Space is one of the 24 kinds of derived-materiality (UpadayaRupa), which is one of the 2 kinds of materiality (the other is primary-materiality, BhutaRupa, which is composed of the 4 elements of earth, water, fire, and air). They're all conditioned dhammas. Only Nibbana is the unconditioned one. Further info. is available at Vism.XIV.34