in large voting systems, it is impossible for consensus to be achieved, cause there will always be someone who doesn't like what the funds are going to be spend on.
So nothing would ever be completed if all parts of the private keys are required for the transaction to take effect.
This is not the best example but it would be like having someone with multiple personalities and all personalities have to agree to do something in order for that to take action...
kinda like Alien X from Ben 10
The best way is having one person call the shots and a counter balance just in case that person starts shooting blanks.