Answer
They should be connected in parallel.
Work Step by Step
When two capacitors are connected in parallel, their capacitances add, and the equivalent capacitance is larger than either individual capacitance.
When they are connected in series, the equivalent capacitance is smaller than either individual capacitance. This is discussed immediately following equations 24.5 and 24.7.
To store the most energy, we want the largest equivalent capacitance, because $U=\frac{1}{2}CV^2$.
They should be connected in parallel.
