is using two UARTs wiser compared to I2C and UART with Arduino + Xbee + BNO055 Sensor
All software serial solutions are much less efficient than HardwareSerial (i.e., Serial).  SoftwareSerial is particularly bad, because it disables interrupts for long periods of time.  95% of the MCU time can be spent twiddling its thumbs, waiting for each bit to come in.

Using the I2C interface for the BNO055 means that the Arduino hardware will independently and concurrently handle each byte of data from the BNO055 and from Serial (i.e., the XBee).  Your sketch can continue to do other things while each byte arrives.  This is most efficient.

Software serial port libraries handle each bit of data, sometimes to the exclusion of all other activity.