Another twin mum here, it's no walk in the park but to be honest I found the newborn stage easiest.
I second a double pillow from asda.
The 2 bouncy chairs were the only thing I bought 2 of
I got a double pram off Facebook marketplace for next to nothing which was amazing.... to be honest, get everything from marketplace, carseats and mattresses are the only thing you need brand new
Had babies in a next to me cot for 6 months then moved into one bigger cot till they started annoying each other (which I got for free from marketplace)
Tommee tippee machine is a big help
Just baby grows and sleepsuits to start as no point getting them changed into anything else
Get your daughter (or yourself) onto some twin mum chats on Facebook. I'm on a WhatsApp group which is amazing.
It's easy to give advice on the 'stuff' and you can get alot of stuff without spending alot of money. The thing you can't help with is the pregnancy and birth but make sure your daughter is well informed on her choices
Best of luck to you