Box 1, a GNS530, tuned to Elstree, 122.400.
Box 2, a GNS430, tuned to D&D, 121.500.
These frequencies are 900KHz apart.
Do you get the same effect if you keep the same difference but use another frequency ie try 121.600 and 122.500.
If so it is something to do with the difference frequency.
If not then I don't know