WT - as far as "spoiling" babies by holding them to sleep etc. Its a tough one. I fed Roh to sleep every night until he was about 8 months old, and then had to spend about a month getting him to settle down on his own without beng on the boob. Maybe that means I spoilt him, but to be honest, although it took a while it wasn't that traumatic to teach him to settle. Saying that I think the general consensus is that babies are capable of self settling from roughly 6 weeks old, which is about when they can be expected to reliably find their own hands/thumbs to comfort themselves with so around then I plan to bring in a routine/ritual for nap and bedtime for settling Freyja down (things like saying certain phrases, dimming the room, the same music on every time etc etc.) In the hope that she'll learn to settle herself. I definitely won't bother until then, and even after I start if she really can't settle I'll step in to help.
Wth crying, again that's personal. Last time I hated hearing Roh cry it upset me too much so I didn't leave him long at all ever. To be honest I'm not much different with Freyja and plus it gives me a headache really fast! I think it doesn't take so long before you can tell the difference between a hungry cry, a pain cry and a grizzling tired cry. The last one I'd leave, the first I'd leave for a short time and the pain/distress cry I'd sort asap.
I did eventually do some controlled crying with Roh, to sort out his sleeping, but not until he was almost a year old. I've no issue with people who do leave babies crying, within reason, but I'm not good at coping with it myself. I'm imaginiing with twins there is often very little choice in the matter, you'd have to leave one or other of them crying at some points?
Hmmm, sorry, not sure that's much help at all, but those are my thoughts