I think you’re underestimating your ability to cope. No one finds early days with 2 easy - but almost all do cope. Sometimes it’s about letting go of certain stuff like housework and concentrating on kids. There is also an element with 2nd one that you don’t focus on them 24/7 like you did with first one …there isn’t the time, as a result you are actually all much more relaxed. You figure that baby is actually ok to not be interacted with every second they are awake!
one other source of help you may want to investigate is a local college teaching nursery courses . They look for short placements with families (usually 1 term) and do prefer families with mixed age groups. The student wouldn’t be with you full time..maybe a few days a week but it will give you a chance to recharge on those days…particularly catching up on sleep. You are expected to “support” the student in terms of allowing them space to complete certain tasks they need to show they can do. Look at local colleges and inquire at ones that do these courses.
I suffered from bad PND with my first dc. As part of a plan to avoid it second time around I investigated this and managed to get a student who came in 3 days a week for around 14 weeks. She started when baby was 3 weeks so it took us up to just at beginning of weaning and end of my maternity leave (it was 6 months then) . She was actually a mum herself of a small child so it was BRILLIANT. I had c- section and always struggled with breastfeeding…she was able to pick up stuff on those days so I could catch up on some sleep, and help with toddler whilst I was wrestling with breastfeeding. She was lovely company and I enjoyed getting involved in some of her assignments.
we also had au pairs when the kids were older and at school. I was working full time then (went back to full time form part time at school age) . We got an au pair to help with school “run”…but actually they walked kids to school and back. Then looked after kids till I got home. That’s worked really well, BUT the kids were older and could explain their needs and tell me if there were any issues.
The au pairs are very young people. Often with no experience of childcare . They’re just teenagers who want to live with a family and attend college to speak English better and do so in return for light childcare and domestic roles.
I would NO WAY leave an au pair with a baby or small toddler. They do not have those skills or experience. Plus the au pair must attend college or language school during the day, maybe not every day but multiple days. They are also staying with you to be immersed in the language- they need to be around you for meals, social time etc to absorb that- not stuck in a caravan alone in their evenings. Add to that their English is pretty basic when they arrive. You have to explain stuff slowly, drop slang, sometimes write it down so they can look up and translate. They need a lot of help navigating their new life for first few weeks like sorting college , sorting transport, advising where they can meet people of their own age for social interaction, explain cultural differences. It actually needs effort on yours and rest of families part. Many agencies impose strict limits on amount of time the au pairs can be asked to “work” in return for board, lodging, hosting and pocket money. They could not be supporting you every day from 9-5 for instance. It’s more like 3-4 hours per day usually form 7:30-9:30 and then 3:45 to 5:45 . I really don’t think this is right solution for you