The answer to this question should really focus on the either proving or disproving the points below and stating a summary or conclusion to your answer which states how you think the evidence more strongly aligns.
The first point would have to be whether it would work at all.
How would you cast a spell from two wands? Would you use them in separate hands or together? You do not need to answer these smaller points in your answer they are just my own thoughts.
The second point would have to be whether it would actually affect the power of the spell
As we all know (or at least I hope you do) the wand is used to focus magical energy so really would we not end up with a larger and less focussed point for magical energy?
Would you actually be able to cast one spell that went to two separate targets?
This would only be apt for using two wands in separate hands but the coordination required for this would be incredible and possibly take years to learn to do effectively at least.
End of points. Now for some discussion.
Clearly you would need two wands that would be aligned to you, thus the elder wand rears it's ugly head again! As Dumbledore would have had 2 wands at one point, Harry also did after repairing his own with the elder wand but that wasn't his style. Dumbledore out of all wizards could have learnt to use two wands although knowledge and skill was more to his liking that pure power.(The hallows excluded) I am going to imagine that I will gain an answer that will just state no wizard in the books was seen to do this so it isn't something that will happen.
How would using two wands work?