They're probably sitting on eggs right now and the babies will hatch soon. Have you noticed whether both parents are flying in & out of the nest very frequently? If so, babies have hatched & are being fed! My young swallows fledged last week and are now zooming about all over the place, so I suspect your little martins won't be long!
I havent been in the garden much all week because of the weather, so I dont know whether they are both in and out. I can see them swooping through the window but not sure if its just one or both of them. When they were building the nest one stayed inside while the other collected bits and bobs. Hopefully it will stay nice over the weekend and I can watch them for a while...it was really peaceful and relaxing watching them build the nest :)