Wa'alaikumassalam warahmatullahi wabarakatuh! Alhamdulillah the first baby's heartbeat is strong, that's wonderful news! Honestly, hearing that the second one has grown and is now sama besar with the first is actually a really good sign, so try not to panic too much right now. A few of my mum friends also went through situations where one twin's heartbeat wasn't detected at the early scan, tapi tiba-tiba nampak later, sometimes it's just the angle of the scan or the baby's position, kan. But sweetie, your doctor is the best person to guide you through this, ask her directly at your next appointment what the lack of heartbeat means, whether it's timing or something else, and what to monitor. Lepas a few weeks if the growth taklah susah, that's reassuring la. Kita do'a yang terbaik untuk both babies, eh! Congratulations on your kembar news!