Cross band repeat mode on the Alinco DR-735 involves use of a 2M and 440 MHz frequency on the radio.  

Testing has revealed that you can access XBR mode ONLY if you have a 2M frequency on the left and the 440 MHz frequency is on the right.  The radio will not go into XBR mode with a 440MHz frequency on the left. 

This is not detailed in the manual for the radio, but this is the way it worked in testing.