Get them tagged so they are easy to identify.  Even different colored between the two even,if possible.  Identification will be important to monitor them. If she does not want one ect it needs to be pulled.  Always let her keep the one she wants.  Company policy is not to let a cow raise twins as we end up with two runts and an open cow.  At the same time with monitoring she can raise one till it does not need milk replacer.  She will run out of milk as they get bigger.  Monitor is the key right know.  Soon as you get additional calves it gets harder.  Good Luck