When I did this with DD1 who was 3 and DD2 as a newborn I would get DD1 set up with her dinner or watching playschool so she was occupied while I bathed, fed and put DD2 to bed. Then once she was down, I would get DD1 bathed, dressed and into bed.

I made sure I had all pyjamas etc laid out and ready to go so everything was fairly slick from that perspective. I think it's just a matter of working out what the 2 kids are doing routine-wise and merging them to suit you.