Two sets of XYZ data columns comparison

[Alllfff]

I have two sets of XYZ data like this
X     Y     Z     and     X     Y     Z
0.1   1     1               0.1    1     -1
0.2   1     2               0.2    1     -2
0.3   1     3               0.4    1     -2.9
0.4   1     4               0.45   1     -4.5
0.5   1     5               0.6    1     -6
0.6   1     6

Now I need to superposition these lines and if possible extract sets of XYZ data for the result line

eg...

    /           
   /             
  /                                 
 /                 
/                          

+

\
 \
  \
   \
    \

=

-----

the problem here is, the X steps are different size which results in different number of rows....ANY IDEAS please?