The problem: You start having multiple classes implementing the same interface. The solution: Create a "Composite" class implementing the same interface the original classes are.

This way you follow the Single Responsibility Principle because the caller should not know nor care whether the class it is calling is one, or many.

public interface INotificationService
{
	public void PushNotification(string message)
}

public class DesktopNotificationService : INotificationService
{
	public void PushNotification(string message)
	{
		// complicated code here
	}
}

public class MobileNotificationService : INotificationService
{
	public void PushNotification(string message)
	{
		// complicated code here
	}
}


public class CompositeNotificationService : INotificationService
{
	private IEnumerable<INotificationService> services;
	public void PushNotification(string message)
	{
		foreach(INotificationService service in services)
		{
			service.PushNotification(message);
		}
		
	}
}