Shetland lambs with their mothers are pretty safe once they're up and suckling. The moments of risk are if second lamb comes quickly before firstborn is up and fed; first time mother either not bonding with any of her lambs or not being good at keeping both lambs close; sickly lamb.
Lambing outdoors you'll be checking frequently, and staying handy once a ewe starts labour, so the only real worry would be a ewe lambing in the night. In my experience, they rarely lamb in the dark - but there are often very newborn lambs when you do your first check in the morning. So - if staying in a caravan in the field isn't an option - make that first check really a first light check.