If your pastures are big or have a creek running through them, sometimes a cow with twins puts one here and one there. The first few days as long as she's nursed, she most likely wont go looking for the second calf. She's being nursed, so you cant tell if both have unless you see them together. If the cow is big and a heavy milker, I keep twins penned up for the first week or two so that the cow knows she has 2 and both calves are past their newborn stage. By then, the calf will go looking for her if they get separated from the other 2. Sometimes it works out and is easy, sometimes its a headache. If it looks like its not going to work out, I take one and bottle feed it and if i have a cow lose a calf, i graft the twin onto her. Works out great if the calf is bottle fed already and a little older than newborn.