You don't.

A test with upm found that an __index method declaring two return arguments is disregarded. They're somehow flattened into the same variable instead, but the normal x, y = func() syntax doesn't work, and x is still the same return type. The comparison of a normal function worked as expected with two return values, so this behavior is unique to __index (and possibly other meta methods).

Fuck knows what Lua actually does in this context, but it does _not_ respond well to __index returning multiple arguments.

Use a table if you absolutely have to return multiple values from __index, or use normal functions instead. As already mentioned, normal functions are able to return multiple arguments. It's just __index that misbehaves.