I agree with the poster who suggested the pack and play, you could use that or have two baby chairs/swings in the hallway. That way nanny could get everyone's coats, hats,etc on and then put the babies in a safe spot while she carries the stroller down. She could position the stroller so the seats are facing the house, get the first baby and buckle them in while still having eyes and ears on the second baby. She can then go back up get the second baby in less than a minute without feeling like she's leaving either of them. This routine can be done in reverse when they get come back home from being out.