"""""""""""Well it should look something like this ... but I can't find a way to snap the red segment on both circle :S """"""""""""

Hey PaQ,
It's going to be a complex calculation. transformation matrices can be created and used on 2 rigid bodies. (the circles work with a SINGLE coordinate system). But this situation brings into play "fixed positions AND the transform matrix). You could look at "Denavit-Hartenberg Notation" for a description of what's involved. So basically you would:

Create a coordinate system for the fixed objects and 2 more free form coordinate systems to plot points with the transform matix (I think its 2 coord systems with that many parts but I could be off by 1)

Then math would pick the points for each constraint coordinate, then plotted to the global "fixed" coordinate system...

lol....:o