I have 7mo twins and they're my first DCs.
Things I would recommend:
Go to a TAMBA course if she has one nearby, I learnt a lot on that day and also made friends with some local twin mums-to-be and we meet up every week
If she works then plan to start mat leave early. Twins usually arrive at least 3 weeks early and from what I can tell most twin mums start their leave no later than 32 weeks (but depends on job, how big you get, complication etc)
Don't do what I did and buy tonnes of clothes unless you live in the middle of nowhere a few baby gros and vests will do, otherwise you'll spend a fortune
Do lots of batch cooking for freezer, also but lots of tinned soup, dried pasta and pesto, cereal bars etc for snacks as you need to eat but won't have time!
Buy two bouncy chairs, essential to either amuse one twin while you feed the other, or to bottle feed them together if you choose to bottle feed
If you are planning to bf then a special bf cushion will be very handy.
I had lots of help at first, with DH off on extended paternity leave plus annual leave for about 6 weeks and I also had frequent visits from DM and MIL (we live far away) and whilst I did feel a bit crowded, it was good to have someone to iron, clean, cook and help with feeds etc
Consider paid help too. I had a night nanny for a few nights a week for a while.
Make sure your double buggy suits your lifestyle. I went for a iCandy peach as its the narrowest buggy and easy to handle, as I live in an urban area and walk everywhere. But it's rubbish for public transport as its heavy and doesn't fold
They can share same cot for at least 4 months if you get a cot bed
You'll need to be super organised. I have a changing station downstairs with nappies, wipes, spare clothes etc. I recorded all feeds and nappies in a book for first 12 weeks otherwise I would forget who had fed when and how long since last poo!
I am a routine queen but appreciate not everyone is. But I got mine in a routine by about 6 weeks and they sleep very well now.
Try to get out every day after the initial few weeks, even if just to local coffee shop as it makes you feel human
Gosh that's a long post but hopefully useful. Will come back if I think of anything more