Hi Damien
The normal behavior is to have two tiddlers open normally, and then move one to the second column with the divert right arrow button at the top of the tiddler.
I am not sure you can set one to open in the second story like that. Saq Imtiaz designed the Stories plugin, which governs that behavior. He would be the one to answer that.
If you have upgraded to 5.1.23 which just came out on Christmas eve, there may be issues. I just got back from a trip and hope to update Stroll tomorrow and publish instructions.