Sorry, I didn't see the bit about twins. As Lily Dust said most clinics are hesitant to transfer more than one embie - our clinic told us that in FS circles they actually see multiples as a "failure", which while I find offensive, I can understand as it does increase your risk of complications.

We were also told that statistically it doesn't increase the chance of "success", it only means that if you do have success it will most likely be twins, ie it's kind of all or nothing IYKWIM?

Our clinic only offered it in strict circumstances, one of which being 3 prior failed transfers. We were offered the chance to try two, and declined. It's kinda good we did as I prefered not to have twins.