Display combined item quantity from multiple branches

I hit a stumbling block and cannot break through.  When I add a product it can be at both branches, if for example theres 2. Now the problem arises when I want to display a product on the store front.  How do I display just one of the duplicate product but the quantity to be combined from both branches and displayed  

I have a table


public class Branch
{
public int Id{get;set;}
public string BranchName{get;set;}
}









public class product
{
public int Id {get;set;}
public string Name{get;set;}
public int qty {get;set}






[ForeignKey("Branch")]
public int BranchId {get;set;}
public string branchname{get;set;}
public Branch Branch {get;set;}
}