I think it depends on the kids. Also-they go through stages of being harder, easier etc. Right now, the 2 kids are probably 1 1/2 times harder than 1 (dd 6 mo, ds 2.4). They play together, and I am more ruthless about mummy time (eg now should probably be "stimulating" dd while ds is off swimming, but really can't be ar$ed so she is enjoying free play).
After dd was born, I would have said 3x as hard. Ds did not especially like dd, and plus he was at a really difficult, negative stage. Dd refused to go in the car, so we were quite limited in where we could go. It was bad.
But I would actually say that 2 kids are more than 2x the fun! Cos they play together, and thats lovely to watch. Also I am so much more relaxed, with both kids-I don't see every foible of theirs as my doing, cos they are so different.
One word of warning though-double buggies are VILE. And suprising expensive for something so ugly.
(both of them are repeated night wakers too btw-it is horrible. However, I have found it easier with dd-not sure why, think maybe I just know what to expect.)