Jovette,
I have not looked there, but a relationship chart is a chart between two people, and it is a blending of their birth dates, times and places, i.e., what one does (and a computer is the only really accurate way to do it) is to take the mid-point of those times and places.
I will give an example. Person A was born on January 1, 1999, at 6:00 AM (and we will assume for the moment they are born in the same place). Person B was born at 10:00 AM on January 5, 1999.
So the chart is cast for January 3, 1999 at 8:00 AM - halfway between both of them. Now, if one were born at 40n00 75 E 00 and the other at 40s00 75 w 00, then the longitude and latitude would be 0n/s 00, 0 e/w.
The author of this technique was Patrick Davidson, and there is a book out on it, and there are others also.
By the way, there is another technique that is similar, called composite charts - and in this case, the planetary positions are added and divided by two. Robert Hand has an excellent book called Planets in Composite which details this methodlogy.
Of the two, the relationship method is the better of the two, for it midpoints time, and not space (which in the cases of some planets there will be some difference because of orbital mechanics which I will NOT go into), and once the chart is done using the relationship method, Rob's book can be used for interpretations.
Dave