Twins here too.
In answer to your question, unless you're on the border of a particular banding as far as SFE is concerned it makes no difference if you have 2 (or more). Even if you are on a border, SFE will lower your income by around £1000 for Child 2 (on the basis of having Child 1 also applying for SFE) which will have very little impact on what level of loan your twins will be entitled to.
In terms of accommodation costs, check out what the policy is and how accommodation is allocated. Some is on a first come first served basis (i.e. as soon as you accept an offer, well before results day), some is allocated after results day e.g. by timed slot. Look on here (and at the Student Room) for likelihood of getting the accommodation you want - obviously if you want say cheap self catered halls but there is a possibility you might end up with expensive catered halls due to the way accommodation is allocated, thats something to bear in mind. Also look at how accommodation is offered for insurance choices in case your DC don't get their firm choices. Accommodation will be the biggest factor / expense in managing 2 DC there at the same time. I agree with @GnomeDePlume - look carefully at contract lengths for 1st year halls. Even at the same uni (Notts for example) some halls are 44 weeks, one is only 31 weeks - makes a massive difference to the overall cost.
Look at cost for 2nd/3rd/4th year. Some unis (e.g. Bath / Edinburgh) are more expensive than say Sheffield.
In conjunction with DC, consider courses with a 3rd year placement - often paid - which can assist with 4th year costs.
Not sure how far you are away from applying but start saving as soon as you possibly can (you and DC). Even a very small amount per week can help. If DC have jobs throughout sixth form, they can potentially save in advance of uni and can maybe return to those jobs in the uni holidays. Agree with others about having a conversation about how this is going to work, what expectations you have for them working / contributing.
Also, get DC involved in meal planning / cooking budgeting. We did a couple of weeks in the holidays where they each had £30 and they had to shop & cook for a week.
Depending on course / location, potentially look at incentives - e.g. Queens Belfast will offer non NI students discounted accommodation, students on some healthcare courses (not medicine though) are eligible for an NHS grant (not loan) of £5k per year.