How does the capacitance of two identical capacitors connected in series compare to that of one of the capacitors?

Don't add, whatever you do. This is how it should be done.

1/c = 1/c1 + 1/c1

1/c = (c1 + c1) / (c1*c1)

1/c = 2c1/c1^2 = 2/c1

c = c1/2 divide the value of one of the original capacitors by 2.

It means that whatever the value of one of the 2 capacitors is, you get the result of the two capacitors by dividing the value of one of them by 2.

It is similar to the result of putting 2 equal resistors in parallel.