Alex explained that they are like two halves of the same whole. As it stands now, they are useless if they are not together. The key difference between Simon's theory and Alex's is that Simon's theory could work as long as they are a team, but Alex's would mean they could split up if it was ever needed. It was known that Lord Death had Death Scythes running the DWMAs in other parts of the world. Would he allow them to go together or would that split them up? Lord Death seems like a nice guy, so Alex is sure that he would allow them to work together as long as it was needed.