At 8 weeks you should be able to hear a heart beat and see it in fact it looks like little flashing lights. So they should have atleast been able to see it but maybe it was covered up by the other baby.
It's common for twins to be covered up by eachother. Also common for two to implant but one doesn't make it or gets absorbed by the other baby. You'll be getting a lot of ultrasounds with twins so you'll probably see both heartbeats at your next one :)
Thank you, they did see what they believe to be the second babies heart but just couldn't hear it