Corrected the original code I wrote per Zool's challenge and the new version will also account for Outfits with built-in hats.
The function just needs to be added to the OnPlayerEquipItem in the Module Events:

//void AntiHatHelmetCombo Function v2, by Seter

void AntiHatHelmetCombo_2()
{
    object oItem = GetPCItemLastEquipped();
    object oPC   = GetPCItemLastEquippedBy();

    if (!GetIsPC(oPC)) return;

    //Check if PC has a Helmet Equipped first and then if the PC chose to also equip a hat OR Outfit with a Hat built-in
    if(GetItemInSlot(INVENTORY_SLOT_HEAD, oPC) != OBJECT_INVALID)
    {
        if((GetBaseItemType(oItem) == BASE_ITEM_CLOAK && GetItemAppearance(oItem, 0,0)== 22 ) || (GetBaseItemType(oItem) == BASE_ITEM_ARMOR && GetItemAppearance(oItem, ITEM_APPR_TYPE_ARMOR_MODEL,ITEM_APPR_ARMOR_MODEL_ROBE) == 168))
        {
            AssignCommand(oPC, ActionUnequipItem(oItem));
            //Error Message that is sent to the PC
            SendMessageToPC(oPC, "The hat doesnt seem to fit on your helm! You think it might be wise to first remove your helm!");
            return;
        }
    }

    //Check if PC has a Hat OR Outfitwith a Hat built-in Equipped and then if the PC chose to also equip a Helmet
    if((GetItemInSlot(INVENTORY_SLOT_CLOAK, oPC) != OBJECT_INVALID && GetItemAppearance(GetItemInSlot(INVENTORY_SLOT_CLOAK, oPC), 0,0) == 22) || (GetItemInSlot(INVENTORY_SLOT_CHEST, oPC) != OBJECT_INVALID && GetItemAppearance(GetItemInSlot(INVENTORY_SLOT_CHEST, oPC), ITEM_APPR_TYPE_ARMOR_MODEL,ITEM_APPR_ARMOR_MODEL_ROBE) == 168))
    {
        if(GetBaseItemType(oItem) == BASE_ITEM_HELMET)
        {
            AssignCommand(oPC, ActionUnequipItem(oItem));
            SendMessageToPC(oPC, "You cannot fit a helm under your hat! You think it might be wise to first remove your hat!");
            return;
        }
    }
    return;
}

Will need to factor in a third option that its part of an outfit, specifically robe appearance.
I'll try to add that section later on.

Add this function to your OnPlayerEquipItem Event Script on the Module Events in order to prevent Helmet-Hat Combo equipping:

void AntiHatHelmetCombo()
{

    object oItem = GetPCItemLastEquipped();
    object oPC   = GetPCItemLastEquippedBy();

    if (!GetIsPC(oPC)) return;

    //Check if PC has a Helmet Equipped and then if the PC chose to also equip a hat
    if(((GetItemInSlot(INVENTORY_SLOT_HEAD, oPC)) != OBJECT_INVALID) && ((GetBaseItemType(oItem) == BASE_ITEM_CLOAK) && (GetItemAppearance(oItem, 0,0)== 22)))
    {
        AssignCommand(oPC, ActionUnequipItem(oItem));
        //Error Message that is sent to the PC
        SendMessageToPC(oPC, "The hat doesnt seem to fit on your helm! You think it might be wise to first remove your helm!");
        return;
    }

    //Check if PC has a Hat Equipped and then if the PC chose to also equip a helmet
    else if(((GetItemInSlot(INVENTORY_SLOT_CLOAK, oPC) != OBJECT_INVALID) && (GetItemAppearance(GetItemInSlot(INVENTORY_SLOT_CLOAK, oPC), 0,0) == 22))&& (GetBaseItemType(oItem) == BASE_ITEM_HELMET))
    {
        AssignCommand(oPC, ActionUnequipItem(oItem));
        SendMessageToPC(oPC, "You cannot fit a helm under your hat! You think it might be wise to first remove your hat!");
        return;
    }
    return;
}

Sunday it is.

Please PM me guys so I know your forum accounts and correspond with you over PMs. Thanks!