Merge 2 PDF into 1 Asp.net MVC

Question

I have 2 pdfs files and want to join the 2 in one file only, my method is as follows:

public async Task<FileResult> Teste(int id)
        {
            var objRelatorioTecnicoData = RetornoTecnicoAssinanteDataById(id);

            var modelPdf = MapperService.MapAsync<IncluirRelatorioTecnicoPdf>(objRelatorioTecnicoData).Result;
            await PreencherUfsMunicipiosPdf(modelPdf);
            GerarUfsMunicipios(modelPdf);
            GerarTecnicoUser(modelPdf);
            modelPdf.TemAutoTermo = modelPdf.AutoTermos.Count > 0;
            modelPdf.DataPublicacao = objRelatorioTecnicoData.DataPublicacao.ToShortDateString();

            const string contentType = "application/pdf";
            byte[] fileBytes = GerarPdf(objRelatorioTecnicoData, modelPdf);
            var numRelatorio = string.Empty;
            var length = fileBytes.Length;
            Array.Resize(ref fileBytes, fileBytes.Length + objRelatorioTecnicoData.DoctArquivo.Length);
            for(int i = 0; i< objRelatorioTecnicoData.DoctArquivo.Length; i++)
            {
                var count = (length + i);
                fileBytes[count] = objRelatorioTecnicoData.DoctArquivo[i];
            }

            if (objRelatorioTecnicoData != null)
            {
                numRelatorio = objRelatorioTecnicoData.NumeroRelatorio;
            }
            return File(fileBytes, contentType, "Relatoriotecnico_" + numRelatorio + ".pdf");
        }

However he prints only the 2nd pdf, I’ve done it in several ways and I couldn’t put the 2 together, someone has some solution to help me?

Thanks in advance.