I reckon you'll be fine and if like my shetland experienced mums the first you will know is when you look in 5 mins after the last check and there are two gleaming white little wriggly faces peeping out at you from behind mum :-)))))
With Shetlands it's not like commercial ewes, if you feed shetlands as if they are all having a single (ie woollyshepherds half a mug max) then they will easily pop out the twins too and have plenty of milk.
Commercial ewes need scanning mainly cos they could have big twins or even triplets, which could mean birthing issues, rejections etc and also often are in a big herd where a bit of extra food for each ewe adds up to quite a bit of cash and where planning who to foster multiple lambs onto is important. Scanning does allow fine tuning of feeding but even some big herds dont bother and for shetlands feeding less rather than more will be fine, as long as they have licks, fresh water and grass/forage.