there are carriers to carry twins at the same time- but those people who have used them say that it takes a toll on their body as they get heavy really quickly
i used to carry one in a sling, but turn the sling to the side and carry the other baby in my arms
otherwise other things people with twin do is - top to toe them in a baby capsule ( but not whilst driving) when they are small ,or put one in a carrier and one in a pram
HTH
odette