I think having a proper bedtime routine might help DS2 (4 months) go for longer than 90 minutes sleep at a time at night, but I'm having trouble working one out that fits around DS1's bedtime and would be manageable on evenings when DH isn't around.
At the moment DS2 tends to nap a bit in the buggy on the way home from school, but will soon wake up when we get in, just after 4. By 6 o'clock he is getting tired but is pretty cheerful as DS1 is in manic entertaining the baby mode. At about 6.20 DS1 settles down to watch some TV before bed. Typically DS2 quickly gets grumbly then and I change him into sleepsuit and take him into our bedroom to feed to sleep from about 6.35. He either zonks out very quickly or wants a long leisurely feed. Meanwhile if DH is around he can take charge of DS1's bedtime - the faffing around, getting undressed, bath, bedtime story etc. takes from 6.45 to 7.15 (if DH in charge) or nearer 7.30 (if I do it).
About 45min to an hour after he's fallen asleep DS2 will wake up again. I'll go in and try and settle him by feeding - sometimes this works, more often at the moment it doesn't. If it doesn't, I tend to get him up, because firstly this tends to be exactly the point when our dinner's ready, and also because I got fed up with spending my evenings in the dark in the bedroom. Once he's up he tends to get a bit grumpy, which suggests to me that it's worth persevering with the early bedtime - if I can just get him to stay asleep!
The difficulty in doing a baby bedtime routine when DS1's awake and playing is that he demands attention / wants to play with DS2 and just generally is not conducive to a sleepy relaxed atmosphere! But lots of parents with two children have clearly got it sorted. Thanks for reading and if you've got any advice I'd be grateful!