I like to have links to Trust in the back matter of Trang, but obviously you can't have retailer-specific links in both books until both are actually up at the store.
I'd heard that Kobo will change the Web address of a book if you update it, which obviously would be a problem: I'd update Trang to include a link to Trust, and then I'd update Trust to link to Trang, but that would change Trust's Web address, so then then I'd have to update Trang again, but that would change Trang's Web address--it would be this never-ending cycle.
To see if the rumor was true, I jotted down the last part of Trang's Web address and then updated it to include a link to Trust (on the theory that if I can only have a link in one of the two books, it should be a link in the first book to the second book). Then I looked at the last part of the Web address, and by gum, it had changed!
That seemed like a really screwy way to set things up, so I opened another browser window, and the address had changed again--but, I realized, it was just the last part of the Web address that had changed. The Web address is this God-awful monstrosity involving two long alphanumerical strings, and one had changed while the other had not.
Which might mean that the Web address doesn't really change every time you update the book--just the last bit changes every time you look the book up. But I don't know for sure, because I didn't write down the entire Web address, I just wrote down the last bit.