Eu escrevi sobre isso aqui: link
TL; DR - O GPO principal ou principal que também é imposto vencerá.
Da Microsoft:
You can specify that the settings in a GPO link should take precedence over the settings of any child object by setting that link to Enforced. GPO-links that are enforced cannot be blocked from the parent container. Without enforcement from above, the settings of the GPO links at the higher level (parent) are overwritten by settings in GPOs linked to child organizational units, if the GPOs contain conflicting settings. With enforcement, the parent GPO link always has precedence. By default, GPO links are not enforced.
EDITAR:
Veja também aqui: O GPO fornece valor inesperado
Lá, ele afirma especificamente:
The Enforce setting is a property of the link between an Active Directory container and a GPO. It is used to force that GPO to all Active Directory objects within a container, no matter how deeply they are nested. The settings within a GPO that is enforced override other settings that would prevail because they are applied later. If there are conflicting settings in GPOs that are enforced at two levels of the hierarchy, the setting enforced furthest from the client prevails. This is a reversal of the usual rule, in which the setting from the nearest-linked GPO would prevail.