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Having stored the files workbook1.xlsx and workbook2.xls locally on my computer I can reference cells between workbooks.
E.g. typing in a cell in Workbook1 the formula "='[Workbook2.xls]Sheet1'!$C$1" to reference a certain cell from Workbook2, Sheet1.
From the projectplace support I learned that this is currently not possible when the workbooks are stored on projectplace.
It would be a useful feature though. :-)