I'd emphatically recommend only having one transferred at a time.
(A) If one embryo is viable and the other isn't, you apparently reduce the chances of the healthy embryo implanting and developing properly. As the body senses something is wrong with one embryo, it is more likely just to abandon the whole pregnancy.
I'll never know whether this is what happened to us during our 2nd round of IVF. 2 embryos put back - one looked good, the other less so. I got pregnant. One developing foetus seen at about seven weeks, with a heartbeat. I then miscarried at 11 weeks.
(B) IVF embryos are more likely to split and create identical twins than in natural pregnancies. That means you increase your risk of triplets or even quadruplets if you have two embryos put into your uterus. These pregnancies can be very, very risky with a high potential for life-long disabilities for any babies that survive the pregnancy.
(C) If both embryos implant - ie you're pregnant with fraternal twins - this pregnancy will still be much riskier than a singleton pregnancy, for you and the babies.
This is what happened during our third round of IVF. When I was about six months pregnant, I attended an information evening at our local hospital for parents expecting twins. To my dismay, I discovered that I was just about the only one who was having a trouble-free pregnancy. From memory, virtually all the other expectant parents were facing problems - some of them very serious. I realised then how risky it had been to put two embryos "back". That's why the HFEA tries to discourage it. .
We were lucky and our twins were born healthy and are now robust ten year olds. Twins are indeed marvellous but I shudder to think how we effectively jeopardised their health by having them at the same time, rather than one after the other.
My very, very best wishes for whatever you decide.