The bottom (negative) plate of C1 has -Q charge and the top plate (positive) of C2 has +Q charge. However, these two plates are at the same node, so they are the same plate essentially.
How can they have a different charge each???
In electrostatics, a conductive material will be an equipotential (the potential will be the same throughout its volume).
That doesn't mean that the charge in the material will be uniformly distributed.
If (like in this case) there's an external electric field, the charge will distribute unevenly. In fact, the charge has to re-distribute for the object to remain an equipotential.
In this case you have a metallic object (two plates joined by a wire), so the potential is the same throughout the object. But some negative charge has moved to the lower plate part of the object, leaving a net positive charge on the upper plate part of the object. This is necessary or else the whole object wouldn't be at the same potential.