Hi @Daan Looij ,
Linking Microsoft accounts is no longer possible, see:
Linking Microsoft accounts
https://support.microsoft.com/en-us/account-billing/linking-microsoft-accounts-4044c1bc-f436-60eb-9f6d-2bf64a7cc47f
What you could do instead is to create an alias and logon with that alias:
https://support.microsoft.com/en-us/help/12407/microsoft-account-change-email-phone-number
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Best regards,
Leon