The first is a pretty obvious one, they're both named Dexter, and a name so nerdy is not to be used lightly by any writer. Then, they both have a lab - I'll let you decide for yourselves which one is creepier, the one with odd machines and creatures whirring about or the one with blood spatters covering the walls.. Thirdly, they both cause chaos and mayhem. For good? For evil? That's that's for you to morally debate on. And it doesn't end there! they both have a highly annoying sister that constantly seeks affection and approval (Even though Debra is way hotter than Deedee, a drawn child character. But on second thought, ewww, lets not go there). Both sisters are the romantic interest of the antagonist. Not only that, but both antagonists share Dexter's secret occupation. Coincidence? I think not!
I'm certain this is just the tip of the iceberg, can you think of any more connections?