.................................................
var dictionary = new Dictionary<KeyValuePair<int, int>, Object>();
.................................................
ExcelWorksheet worksheet = package.Workbook.Worksheets[2];
var cells = worksheet.Cells;
dictionary = cells
    .GroupBy(c => new { c.Start.Row, c.Start.Column })
    .ToDictionary(
         rcg => new KeyValuePair<int, int>(rcg.Key.Row, rcg.Key.Column),
         rcg => cells[rcg.Key.Row, rcg.Key.Column].Value);
.................................................

.................................................
//Пример заполнения ячеек
KeyValuePair<int, int> keyValue;
for (int j = 0; j < dictionary.Count; j++)
{
    keyValue = new KeyValuePair<int, int>(j, 3);
    worksheet.Cells[j, 1].Value = dictionary[keyValue];
    keyValue = new KeyValuePair<int, int>(j, 4);
    worksheet.Cells[j, 2].Value = dictionary[keyValue];
}
.................................................

System.Collections.Generic.KeyNotFoundException: The given key '[0, 3]' was not present in the dictionary.
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at kern.Controllers.ExcelController.Post(List`1 files, Double R2) in C:\Users\vvf6426\source\repos\kern\kern\Controllers\ExcelController.cs:line 154
   at Microsoft.AspNetCore.Mvc.Internal.ActionMethodExecutor.TaskOfIActionResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at System.Threading.Tasks.ValueTask`1.get_Result()
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeActionMethodAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeNextActionFilterAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Rethrow(ActionExecutedContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeInnerFilterAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeNextResourceFilter()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Rethrow(ResourceExecutedContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeFilterPipelineAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeAsync()
   at Microsoft.AspNetCore.Builder.RouterMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.StaticFiles.StaticFileMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)