Function two_product_2presplit

pub fn two_product_2presplit(
    a: f64,
    ahi: f64,
    alo: f64,
    b: f64,
    bhi: f64,
    blo: f64,
) -> [f64; 2]

Same as two_product where both of the inputs have already been split.

Avoids redundant splitting.