I've never used Push Sharp, but based on this code :
You are currently using this method, which accepts a single Registration ID :
public static GcmNotification ForDeviceRegistrationId(this GcmNotification n, string deviceRegistrationId)
{
n.RegistrationIds.Add(deviceRegistrationId);
return n;
}
Use this method instead, which accepts multiple Registration IDs :
public static GcmNotification ForDeviceRegistrationId(this GcmNotification n, IEnumerable<string> deviceRegistrationIds)
{
n.RegistrationIds.AddRange(deviceRegistrationIds);
return n;
}