If you're using the standard "Import Ledger File" option, and aren't supplying the URN in the CSV file, each line of the journal will given a separate URN, so they won't actually be imported as one journal, just as separate one-sided transactions.

It's better to supply the URN in the CSV file - as long as you don't use URN's that Sage may use!

Or use a better import... (Excelerator?)

As Geoff Says, it's better to supply a URN in the Import file, I suggest using 99 in the URN field on all the lines associated with the NL Transaction Posting file. It doesn't matter what number is used providing it is the same on all lines, this number is NOT posted as the transaction is re-assigned the next system URN from the correct sequence. This is from the comments in the NominalTransactions.xls example file:

"Use this to group matching transactions, by entering the same value for each transaction in the group. Transactions with the same value will be assigned the same new generated transaction URN during import. The value in the import file is not used as the actual transaction URN, as that is generated automatically."

Take a look at Excelerator for Sage 200:

https://www.youtube.com/watch?v=hATZ2bnuPKQ&t=6s

It takes care of all the control and validation required by Sage 200. So no worries about URN's etc.