In the books, wands aren't just objects. They have a degree of sentience and therefore can conceivably rebel against the user. I imagined there is a compatibility issue with using multiple wands at once that may have been temporarily suspended when Harry used them on Fenrir. Might be cause the wands hated Fenrir too or liked Harry enough to let that one spell go. I don't know. But in my own head canon the use of multiple wands is untenable because it just doesn't work out and the wizard is more likely to explode themselves that fire off a good powerful spell and Harry, as always, just has really good fucking luck.