No experience of more than one child, but have read others say that second lo tends to fit around first one's routine.
It's unfashionable on mumsnet but we started a routine early mainly due to circumstances, with bedtime and milk established by 6 weeks and nap times by 8 weeks. Daily baths began from 2 weeks at 9.30pm because it was the only thing other than breastfeeding that stopped him from crying (seriously, that 5 minutes of silence while he was in the water was bliss, he cried immediately before and after you put him in). This came to signal bedtime and was brought forward over several weeks/months to 7pm. He's now 7 months and the bath seems to be the highlight of the day.
Feeding routine was established after I stopped bf at 4 weeks. LO had a missed tongue tie and lazy latch, I was at the end of my tether and he was constantly hungry and tired (wouldn't sleep because he was always hungry). When finally diagnosed at a bf clinic, we decided to move to formula. The trouble was that he no longer gave any feeding cues, he goes from 0 to screaming the house down in 0.1 seconds. I think he felt that his cues were being ignored because he was always hungry, despite bring attached to my boob for most of the day and night. So we saw how far he could last between bottles a couple of times and then based the routine on that (3-4 hours). The only time he screams for milk is early morning, and I would rather feed him before he gets that screaming point during the day.
Finally, he's a sleep fighter and would be chronicly overtired in those early weeks. A family outreach worker told me to look out for cues so I spent a week or so with him at home and watched him like a hawk. He would sometimes yawn after 1.5 hours of awake time on the dot, so I started to rock him to sleep 1.5h after his last nap, even if he wasn't yawning.at around 4 months it stretched to 2 hours, and even now it's 2-3 hours depending on the time of day and what we're doing. He doesnt often sleep in the car or pushchair so this awake time limit dictates how long we spend out of the house.
That was long. My overall point is that sometimes babies benefit from routine early on, especially if it's not always obvious how/when to feed on demand or if they don't fall asleep easily and stay awake for hours if you don't encourage sleep (and then scream at you for hours more due to overtiredness. ..)