Properties of reciprocal Quines
I don't see why they should have the same length, given that one can succinctly output large amounts of whitespace using something like

for i=1 to 1000 do
    print " "


So, given any mutual Quines $A$ and $B$, one could surely add 1000 spaces to&nbsp;$A$ and 2000 spaces to&nbsp;$B$ and adjust the programs to use for loops like the above.