I'm going to be honest and say that the biggest challenge with having two, is the floor time. Tummy time, just laying on the floor, playing with toys on the floor, etc... It just doesn't happen as often with Emie because I'm always afraid that Coen is going to clock her with a toy or just trampler her because he still doesn't always know that she exists.
I miss hanging on the floor with a baby though and while I try to do it at night when Coen is in bed, that is her sleepy time too and she's not always in the mood. We're getting it in more and more, but not as often as we did with Coen and I'm sad about that.
Emie's most awake time is right after she's eaten and I cannot always lay her flat then, because she's been known to throw up everything because she's laying flat and it just doesn't feel good on her tummy. So, we usually wait until she's settled and maybe rested for a minutes before we give it a go, but it's seriously not that often.
We've found where recently, she's lifting her head forward, like doing crunches, when she's in her bouncy, infant seat or even her swing. It's pretty funny and we're not sure where she's trying to go or if she's really doing crunches, but either way... we know that she's building neck strength. We just have to work on the back side too and get her to start lifting her head more. She tries very hard, when we do put her on the floor and most of the time does just fine, but we worry when she doesn't come up for air and is just rubbing her face on the blanket. At that point, we usually just flip her because we can tell that she' just not digging the activity.
Either way, we are GUILTY of not being on the floor (with her) like we were with Coen. It might be a "second child" issue, but I still feel bad about it and hope to do it more in the upcoming days/weeks.