Detailed Instructions: Given a travel related situation, and given a specific question about the situation, summarize the question into a title
Problem:Someone I know is a Swedish-South African citizen, and has a Swedish ID card and a South African passport.

He's going on the Allegro train Helsinki-St Petersburg (and back), and his documents, combined, are sufficient for this trip (Swedes have freedom of movement in Finland, while South Africans are visa-free for Russia for 90 days).

The problem is: when booking the ticket online, you have to enter document info to be transmitted to the Rajavartiolaitos and the FSB, and the system only lets you put one document per direction.

So the question is: should the person put the document to be used at Finnish, or Russian, border control?

Does it differ in each direction?

I e-mailed and called VR (Finland's railway operator), but they were completely bamboozled and ignorant about the situation, and thus of no help (they said "put in the document to be used during the journey")

Also e-mailed the Rajavartiolaitos head office, in Swedish followed by English, but haven't received a reply thus far. Would write to the FSB St Petersburg division as well, but my Russian is too poor and I doubt they'll reply to a message in English.

Solution:
What travel document should a dual national register when booking the Helsinki-St Petersburg train?