I did a 6 hour flight (which was a 14+ hour day) with my son when he was 8 months. It was pretty easy. Every child is different though. If he was a difficult baby/toddler, there's no way I would have attempted it by myself.

The twins question is easy. You're only allowed one infant per person. So, if you have twins you and the other adult both have a bag for food, changing mat, wipes, diapers, etc.

If you're already thinking it's too much to spend a few extra bucks to improve the experience for everyone, you probably shouldn't be flying anyway.