I love twins so I would keep both calves in the herd as long as they do well! If the pasture in your area allows the cow to produce enough milk for them to thrive and her to rebreed you will make good money.
If the cow has to cover several acres to live the twins can be a problem.
Pictures like this mama, makes ya want to have some Herefords. I love Black Baldies and ought to be having my first bunch of black baldies sometime this fall during calving, after A.I. I turned out a Hereford bull on my Registered Black Angus Cows