I'm a little late to the party, but I just looked up this guy's backstory and it's actually kinda tragic... Poor Steven Boxleitner... (Yes, that's his real name...)
Basically, from what I've read online, a mind-reading experiment with a sapient and sadistic albino lab mouse named Squeaky went horribly wrong, and merged the two together, creating Two-Brains. In the original shorts, he had some sort of a Jekyll and Hyde dynamic that was downplayed and kept subtle in the main series. (I get why they put it aside, but I kinda would've preferred that whole split personality dynamic.) Poor bastard...
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ErikaFuzzbottom
Was not expecting to see fan art of Dr. Two-Brains on this site, but I approve.