What is the meaning of the Japanese surname Nijo?

This might be helpful, the following resource suggested that the words OKAN’ NIJO mean ‘ONE’. Perhaps the surname's meaning is a derivative thereof?

http://www.frommers.com/destinations/japan/in-depth/language

The most popular result is Nijo Castle, where the jo part means castle.

https://en.wikipedia.org/wiki/Nij%C5%8D_Castle

Then add Ni which is an indirect object marker also meaning at or in, so may well be at the castle. http://japanese.about.com/od/japanesevocabulary/fl/Particles-Ni.htm

(satguru)