using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var tabela1 = new [] { new bpai { cnes = "A", pa = "1",  quant = 2 }, new bpai { cnes = "B", pa = "1",  quant = 4 }, new bpai { cnes = "C", pa = "2",  quant = 80 } };

        var tabela2 = new [] { new bpac { cnes = "A", pa = "1", quant = 2 }, new bpac { cnes = "B", pa = "1", quant = 4 }, new bpac { cnes = "C", pa = "2",  quant = 80 } };

        var union = tabela1.Select(a => new { a.cnes, a.pa, a.quant })

                    .Concat(tabela2.Select(b => new { b.cnes, b.pa, b.quant }))

                    .GroupBy(x => x.pa)

                    .Select(gp => new

                    {

                        Pa = gp.Key,

                        Total = gp.Sum(x => x.quant),

                        Itens = gp.ToList()

                    });

        foreach(var u in union)

        {

            Console.WriteLine($"Pa: { u.Pa } - Total: {u.Total} Qtd Itens: { u.Itens.Count }");

        }

    }

}

​

class bpai {

    public string cnes;

    public int quant;

    public string pa;

}

​

class bpac {

    public string cnes;

    public int quant;

    public string pa;

}