In this case you can only link one of them. Multiple accounts per service is not supported.
It's not a typical use case I would say, pretty sure most kept using the same account from Xbox 360 onward, but regardless we might support it in the future. It's not as straightforward to implement here considering it would be problematic for the leaderboard if there are no restrictions on linking multiple accounts. Most likely if we did allow it, only one account could count towards the cross platform ranking.