The output impedance for the CY2305 is between 20-30 ohm. We recommend using a 20-30 ohm series resistor to match a 50 ohm transmission line.
Ideally, you should terminate your unused output to equal your other outputs. For example, if your other output load is 15 pf, then you should terminate the unused output to 15 pf. But it is also okay to leave unused output float.