Physical changes caused by charge pump
No current flows, and the number of electrons on each plate remains the same. So now you have 2V across the new capacitance.

One view of the question is, how does the new voltage 2V across both capacitors square with the same charge Q on the top plate of C1, and the equation Q = CV? There does seem to be an inconsistency.

However, this is resolved by the formula for series connection of capacitors 1/C = 1/C1 + 1/C2 so the the capacitance has changed to C/2.
So with the same charge Q available from the resulting capacitance C/2, the voltage across it must be 2V.

This also squares with the conservation of energy E = 0.5 * C * V^2.
With both C1 and C2 storing energy E, the result of connecting both capacitors in series is 2E.