As a Danish resident, you are only allowed to have one aktiesparekonto, and there can only be one owner of an aktiesparekonto, i.e., it is not possible to link an aktiesparekonto to a joint account.
Any aktiesparekonto beyond the first one established will be treated as a regular account for tax purposes, and there will therefore be no tax advantage in opening multiple aktiesparekonti.
If you wish to invest your funds through an aktiesparekonto with Saxo Bank but already have one at another bank, you must close your first established aktiesparekonto before opening a new one with Saxo Bank.
Please note that it is not possible to transfer an aktiesparekonto to Saxo Bank.