[jb1991]

I mean, from a technical level, it’s hard to argue that playing a single voice, or single hand, requires less effort than sight reading multiple voices or two hands simultaneously.

[Hackbraten]

From a technical level, you’re obviously right.

But when sight reading, there’s a lot more going on than just technical skill. Depending on the person who is sight reading and which piece, playing both voices at the same time can, on an intellectual level, help tremendously with comprehension that it more than offsets the additional technical burden.