Sparky in the HT forum is pregnant with identical twins and they totally missed the other baby on her first scan. I remember asking her where she was going to go for her first scan (as we live close to each other) and she said just the local one, they just need to check HB, well she then realised that maybe going to a better place is better. Not all sonographers are the same. I think it is a really good sign that she was able to see something in the second sac and that hopefully she just wasn't as good at getting the HB.
Good luck on Monday, I have my anatomy scan then also :)