I have twins. They were my 3rd and 4th. So in terms of equipment I had to buy an extra cot, Moses basket, car seat and a double buggy. And a bigger car! Hand me down clothes were fine as they're all boys.
So apart from that it wasn't too bad - there was The sure start maternity grant at the time (they're 15 now so not sure if that's still a thing?) so that basically got spent on the buggy.
Then it was fine until they were teenagers and 2 x secondary school uniforms was hard (no supermarket cheap polo shirts, it's blazers etc here) and the school trips x 2.
I also have a disabled son which means I can't work, and am now a single parent.
In a way that means I'm lucky as they get pupil premium so some school trips are free.
And they went on scout camp but I only had to pay half the price as they applied for a hardship grant.
They're not into designer clothes and don't really ask for much in terms of Christmas/birthday presents. They are good boys! 
But if I had them and not my son who was disabled I'd be able to go out to work. I don't have to take them to school, my 17 year old still has to have me take him, he can't be left at home on his own, unlike his younger brothers etc.
So we'd be better off despite there being two of them as I'd be earning a lot more than Carers Allowance pays me (£66.15 a week, woohoo, so rich!)