You can use the digital (orange) ports on the back of the CDJ900 and the DJM900 to connect to each other, using a single digital coaxial cable between each device.
The cable you'd need is as per this example.... http://www.avreview.co.uk/review/reviewproduct/mps/RPN/3147/prod/QED-Qunex-P75-Digital-Coaxial-Interconnect/RCN/42/rgn/5/sp//v/5