Okay. Pat your head and rub your stomach at the same time. Not bad. Now hop on one leg and sing 'Daisy Bell (Bicycle Built For Two)'. Good job. Now "make sense of a message composed in one language while simultaneously constructing and articulating the same message in another tongue". Not so easy.
This 2014 article in Mosaic Science
delves into the neurological explanations of how a human being can perform such a complex task – as well as some of the more practical challenges of having such an ability as a profession. As incredible as simultaneous translators are, they're still fallible. In short: sometimes "frozen bull's semen" comes out as "deep-frozen sailors".
Gaffes aside, the task involves almost countless sensory and cognitive elements. It's an ability, furthermore, that has no obvious evolutionary reason for existing. One that makes simultaneous translators the rarest birds in the great forest of neuroscience. Birds, furthermore, which could be a key to translating the confusing puzzle represented by our very own brains.