RC time constant in circuit with two caps with different potential
Use superposition.  The total voltage at a given point is the sum of the voltage from each individual voltage source and current source in the circuit.  

When calculating the contribution of a selected voltage or current source, other voltage sources are shorted and other current sources are left open.

For example, in the circuit below (from the link), Vo = Vo1 + Vo2 .



You should also add some series resistance to your model to get a physical result in the case where the 1.8V source is active and the 3.3V source is shorted.