I struggled with this situation when DS2 arrived and DH went back to work (12hr shifts 7am to 7pm) and drafted in help from my folks for a few weeks. DS2 was very demanding with feeds (typical of a BF newborn).
After a couple of months things get easier, as your baby's routine starts to become predictable (feeding times, bathing etc).
What I usually do is put DS2 in the cot whilst I run the bath for DS1 (around 5.45-6pm) and then I bath DS1 (wash hair, face, body etc, brush teeth) takes around 5 mins, then I pick up DS2 from the cot and sit in the bathroom whilst DS1 plays for 10-15 mins (if DS2 is hungry I feed him in the bathroom whilst supervising DS1).
Once DS1 is ready to get out the bath I put DS2 back into his cot for a few mins and then dry DS1, get on PJs. I go back into my bedroom to pick up DS2 again, and then we all pile into DS1's room for a story or two, bedtime kisses and cuddles, and then I feed DS2 back in my own bedroom. DS1 quite happy playing with his Lego Duplo whilst I feed DS2 (by this time it's usually approaching 7pm).
It is tricky some nights, and can be quite stressful (DS2 is now nearly 6 months old, and DS1 is 3.8yo), but am relieved when DS2 falls asleep around 7pm-ish, and it's usually only DS1 awake til 8pm which he knows is lights out (and he is usually fine about switching out his own light by this point as he's knackered anyway).
It is getting easier as the months have went on, takes a lot of getting used to, esp if there are nights when you have to just get on with it by yourself with no other help.
When DH is around, the routine stays much the same. Just much easier for me as I've got some help for a change!