It is quite clear that an investigator can only use one item that gives the highest bonus in combat, even if they have multiple items (i.e. Multiple weapons)
However, what about spells such as Wither? We must first test Lore to see if we get a bonus and then flip the card and fulfil additional effects based on our results. Its possible that you don't get the bonus due to failing the test.
Does this count as the component that you used for combat and you cannot use any other items, even if you failed? Or is it special since the spell doesn't give a plain combat bonus like regular items?