First of all, don't you mean capacitors instead of condensers? Secondly, consider the first free electron in the middle to be set in motion. Because it is attracted by the outermost right hand plate charge (+Q) and pushed away by the outermost left hand plate charge (-Q) due to the respective Coulomb forces, it will move to (b). Because of the fact that this electron was detached a positive ion emerges, so a +e will move to (a) for the analogous reasons. The second electron will move to (b), but the force driving it is now somewhat weaker because there is one -e in (b) pushing it away and one +e in (a) attracting it. The third electron experiences an even weaker force because there are 2(-e) in (b) and 2e in (a) now, and so on, until (b) gets a sufficient number of electrons to add up to -Q and (a) +Q, when the total force dies out.
Because the dielectric will polarise due to the mechanism described above and a +Q -Q +Q -Q concentration will form, so it's like two capacitors in series.