Hi there -- congrats on your twins! I have twin 15 month old girls. I would sign up for care.com and scroll through their postings -- we've only used sitters through there for occasional childcare and not for full-time nannying, but it is definitely worth looking at as a possible resource.
In re: au pairs -- we have our first one arriving later this month (we had a nanny for the girls' first year). As far as I am aware, au pairs come through licensed services that take care of all of the admin aspects, including visa requirements. We are using Cultural Care, but I know there are several other agencies operating in this area (Au Pair Care, Au Pair America, etc.). There can be a decently long lead time with getting an au pair, but there are always people looking to rematch with a new family, stay an extra year, etc. If you are interested and expecting so soon, it is probably worth googling these agencies and reaching out now. I should also say that our current nanny had no experience with twins, and it has worked out fine -- I had no prior experience with twins either
Congrats again!