Sorry you're having to go down this path.
I think generally they do not transfer more than one except under certain circumstances - like age and multiple failed cycles. Having more than one transferred doesn't necessarily increase the chances of success, but it does increase the chance of twins or higher-order multiples, which is a significant risk for mother and baby.
That's exactly what we were told through our clinic.

One very important thing to think about when considering transferring more than one embryo is how many babies you would actually be happy to carry and have. Each single embryo has the chance of splitting into two naturally. Imagine if that happened to both transferred embryos and you ended up pregnant with 4 babies. What would you do? There is then a much higher risk to you and the babies, and in that situation many doctors would start talking about selective reduction...

Yes it's unlikely, but it's food for thought.

Good luck with your IVF journey. It can be a tough one, but when successful it is more than worth it