The upcoming books about Numair Salmalín will come in a duology, like the Trickster Series. The Gift of Power, the first in the duology, will be published on a date yet to be announced. According to Tamora Pierce, it will cover Numair's years at the University of Carthak, his meeting Varice Kingsford and Ozorne Tasikhe.
Further information can be found on her tumblr: http://tamorapierce.tumblr.com/
Update as of 2016:
They will be a trilogy, beginning with The Gift of Power, and followed by The Exile's Gift. The third and final book of the trilogy is as yet untitled. The publishing date of the first book has been pushed back to summer of 2017 with the exact date of publication yet to be announced.