There's very little reason not to open multiple workbooks in Excel. Key lines of code are:
Application.EnableEvents = False
Application.ScreenUpdating = False
...then you won't see anything whilst the code runs, and no code will run that is associated with the opening of the second workbook. Then there are...
Application.DisplayAlerts = False
Application.Calculation = xlManual
...so as to stop you getting pop-up messages associated with the content of the second file, and to avoid any slow re-calculations. Ensure you set back to True/xlAutomatic at end of your programming
If opening the second workbook is not going to cause performance issues, you may as well do it. In fact, having the second workbook open will make it very beneficial when attempting to debug your code if some of the secondary files do not conform to the expected format
Here is some expert guidance on using multiple Excel files that gives an overview of the different methods available for referencing data
An extension question would be how to cycle through multiple files contained in the same folder. You can use the Windows folder picker using:
With Application.FileDialog(msoFileDialogFolderPicker)
.Show
If .Selected.Items.Count = 1 the InputFolder = .SelectedItems(1)
End With
FName = VBA.Dir(InputFolder)
Do While FName <> ""
'''Do function here
FName = VBA.Dir()
Loop
Hopefully some of the above will be of use