Qlikview creates kind of an index (not in a literal sense) on each field leading to more memory consumption per field. Hence fewer no. of fields is recommended. Secondly, null values slows the calculation time and its advised to avoid them as much as possible.
Having said that, in this case lets say user have option to select document no. By having one field the selection is made for all three transactions which is not possible with multiple fields. Its better to have same column for all transactions.
if you rename and concatenate both tables are displayed as one table , but the problem could be u wont have any assocaition with the concatenate tabe.
if u keep the names as same then both tables are joined and concatenated and displayed as a single table, where u have the asoocaion between the two tables,
depening on ur requirent which one would be better for you