I'm no expert by any means, but I've always heard that orphaned calves are quickly adopted in most elk country (cold places).
(Where I live it is a little different. One study says that orphan elk in warm climes often become loners much more so than previously thought. Especially in places where they don't need to herd up for survival).
Anyway, the reason calf calls are said to be popular is because cows will respond to rescue an orphan.
Twin elk are said to be very rare, much more so than twin deer. I suspect you may have seen a calf that had already been adopted by a wet cow.
The rule of thumb I've always heard when facing the calf/cow dilemma is take the sure shot - whichever it is. Orphans will be adopted and calves are often as big as deer.
