It definitely is in the source, since the Behemoth Shield always has been the only one equippable with a 2h weapon.
I suggest looking at CMover:: DoEquip, especially looking at IK3_SHIELD.
Probably everything after this section:
Code:
if( pItemProp->dwItemKind3 == IK3_SHIELD )
{
if( pHandItemProp && pHandItemProp->dwHanded == HD_TWO )
{
if( pInventory )
{
CItemElem *pItemElemOld = pInventory->GetEquip( PARTS_RWEAPON );
if( pInventory->UnEquip( PARTS_RWEAPON ) == FALSE )
return FALSE;