I've known several outcomes from double transfers. A few have had twins, I know of one set of triplets and a lot of singles. Unfortunately there are also a lot of negative results as well. You need to go into it feeling that you can cope with any outcome.
I'm so happy not to be going down this road any more. My first son was the result of a cycle that I was advised to cancel due to poor response. However it was to be my last before trying a donor, so I was incredibly lucky that one of the two eggs collected resulted in a baby.
I just feel that it's something no one can predict and all FSs and patients have different opinions and experiences. At the end of the day you need to be comfortable with thd choices that you make and feel that you have the support of your clinic.