Ditto MF
If there is a need to intervene (and being first time gilts there might be) it's fairly impractical to try getting into a dark arc with a stressed mammy and I don't know many vets who'd be prepared to do it. In a shed you can easily check on them and get in (and out) quickly and easily
Once the danger period of the first 2-3 days is past there's no reason they can't go out into an arc, but I'd always farrow indoors where possible.
If you are breeding pedigree you'll need the litters separate until you can ID them, there's also no guarantee that both will farrow together and if they're all in the one arc you're going to lose piglets to squashing
Most first time gilts are fine but you do get some that freak and try to kill the piglets as they associate them with pain - you wouldn't want to try sorting that out in an arc.
It's going to be a steep learning curve - but you've plenty time to get organised. The GOS breeders club has some good articles about breeding - if you've not read them, I'd start there.
HTH and good luck!