I think this is what your looking for but I could be wrong,
in VBA to open an existing excel file you can use the following code

Name = "File Path"
workbooks.open Name

Within your program to negotiate between open workbooks

workbooks("Name of Workbook").Activate

works pretty good, you can even string a worksheets on so

workbooks("XXXXX").worksheets("XXXXXX").activate

you can use numbers instead of names with the workbooks and worksheets
commands, i.e. workbook(1).worksheet(1).cells(XX, XX).value but I find I get
lost, so I prefer to use the actual names, hence ("XXXX")