Dear,
If any Condition Type ( more than One condition Type in Sales Order ) having Field " Condition Class is Prices " then second condition type will be inactivated automatically .This is standard sap .So,you dont need to add same condition type twice in sales order .Just change the price manually of that condition type in Sales Order If It is Manually Condition Type .
If you have any doubts please let me know .
Thanks,
Naren