Attraction between 2 rigid bodies I want to make two rigid bodies attract one another. 
But I don't want them to attract the other rigid bodies. 

In other terms I want in my scene : 


Rigid Bodies A and B attract one another and get closer and closer. 
Rigid Bodies C and D attract one another and get closer and closer. 
But A has no influence at all on C and D.
B has no influence at all on C and D. 
C has no influence at all on A and B. 
D has no influence at all on A and B.


How coud I achieve this ? 

Chears
Forces only have an effect on rigid bodies on the same Layer - so use separate layers for each set of independent objects and forces.

Essentially you need to do the following :

On one layer, create objects A and B. Create a 'Force' force field at the origin of object A (select object A, press Shift-S and select 'Cursor to Selected', then Add/Force Field/Force) and parent it to object A. Create a 'Force' force field at the origin of object B and parent it to object B. Set each of the force fields to a negative strength (eg, -50). You should now find that A and B are drawn towards each other.

Swap to the next layer and repeat for C and D.

Running the animation (with both layers selected) should now result in two pairs of objects attracted to each other but not the other pair.