I'm an MB w/ twin toddlers, our nanny started with us when they were 4 mths old. They were our first kids.
If you already have a set of standard questions then I wouldn't add a ton more. You can ask them what their main concerns are, what is most important to them, etc...
Do you have twins experience? If so, you could be very helpful to them. If not, there are a couple of things to think about with twins that are a little different than singletons (or maybe not so different but perhaps more important or challenging).
For instance, with twins a big decision is whether to maintain a fairly firm schedule and keep both twins on the same schedule (assuming no medical issues requiring variation) for feeding, sleeping, etc... This is a very individual decision but the nanny and parents have to take the same approach. I didn't know enough to think this through fully before my twins were born, but I'd made a decision and had a definite approach by the time my nanny started and it took us a while to be on the same page. I wish I'd had the foresight to discuss it with her when we were interviewing.
Also, if the mom is planning to breastfeed (which can be challenging for anyone) that can be trickier w/ twins for a host of reasons. You might not be involved soon enough to be an active support for that, but maybe you would be. It's such a personal thing, and can be really tough on the mom w/ twins, so it might be nice to show in some way that you're flexible and supportive of whatever they decide (assuming you are) or whatever ends up working.
With twins a due date is very subject to change (early delivery). Have they considered whether their start date would change if the babies come early and how they would want to handle that (or what you might be able to accommodate).
Are they connected w/ a local parents of multiples club? ENORMOUSLY helpful.
I think discussions of whether they'll be dressed alike are pretty irrelevant for now, and totally up to the family anyway. It's the stuff in the early months that can feel overwhelming with twins where you could really be helpful. Have you dealt with any preemies? Their twins may well go to term (which is 37 weeks for twins by the way) but if they come early there could be some sensitivities around feeding challenges, developmental milestones, etc... If you have any experience w/ those sorts of things it would be worth mentioning.
They probably will have ideas of their philosophy on things like never letting a baby cry that may well change when faced w/ two simultaneous screaming babies. So I think the best thing you can do w/ first time parents of twins is be able to be calm, flexible, and supportive.
Good luck w/ the interview!