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.
