Spells and Items during combat
From the reference guide, page 11:


  He may use one effect that provides a bonus (for example “Gain +3 Will”). If multiple effects provide a bonus, he uses only the highest bonus.


This wording implies that when multiple effects are available he uses the highest bonus, not that he strictly uses the effect with the highest bonus potential. It implies to me that all the effects are in play and overlapping.  Narratively it supports the idea that multiple items operate concurrently, an idea also supported by the fact that items that grant rerolls can accumulate. This is how we have been playing.

Note that choosing to use Wither comes with its own cost when it fails, so there is already a penalty when it is attempted and failed.