I would try several different things:
1. Separate them again.
2. Make India MOVE HER FEET every time she starts up with her calling. Tell her no sharply and then circle her quickly. When Whiskey started up calling Scooby I would tell her no sharply, bump her, and circle. Then back to business until she called, then I would repeat. She stopped calling for him. (This didn't work for Scooby though, because when I rode him he rarely called out - he just had issues when she left him).
3. Make the horses work hard when they are together, and relax (except for the above) when they are apart. So at the playday when you are together, circle both of them, sidepass, whatever, just don't let them relax. Then when you alone or with other horses, let them stand quietly (in case they call out, in which case do number 2 above).
I feel your pain - Scooby had major seperation issues, would scream his head off at the shows when separated from Whiskey - I had to stall him at the other end of the barn, as far away as we could get him. I tried to keep hay in front of him and keep him content, but he would work himself up. He was all business in the arena, but it was such an irritant I ended up selling him - but made sure his new buyer knew his issues. I hope you have better luck than I did :)
There is no passion to be found playing small - in settling for a life that is less than the one you are capable of living. - Nelson Mandela