I have 2 years 3 months between mine. I think when DC1 hit 2y7m everything got a million times easier as he had a language breakthrough (ASD, was mostly nonverbal until then). I was a solo parent when DC2 was 6 weeks old until she was 8 months, and the things that helped were:
Double buggy. Don’t put pressure on yourself to get DC1 walking places when you’re out alone with two.
Prioritise DC1 for a cuddle/attention etc unless DC2’s issues are urgent. DC1 needs to know nothing has changed between you.
I got DC1 a baby doll to put him in the role of carer not baby. He didn’t take to it as well as I’d hoped but it did help him process.
I treated DC1 like a baby when he over-egged things for attention because he’d seen his sister getting cuddles etc in a way that makes it clear we are pretending and he’s really a big boy. I also shut down any attempts to push her etc very swiftly.
The hardest bits were night feeds, so I cosleep with both DCs, and bathtimes, so DC1 goes in the bath, occupies himself while I dip DC2 in the bath for a quick splash then I dry her on the landing so I’m never out of sight of DC1 when he’s in the water.
Make everything as easy as possible and don’t worry too much about getting everything perfect. Eat lots of snacks to keep your energy up. My two adore each other. 💖