Is it possible to link multiple Out-of-Home user accounts to the same Daikin Altherma LAN adapter ?

No, at the moment we only support multiple Daikin Altherma LAN adapters to be linked to 1 Out-of-Home user account. You can always remove the Daikin LAN adapter from your Out-of-Home user account, and link it to another Out-of-Home user account.