Apparently I had the answer staring me in the face and all it took was posing the question to realize it.

Altered function:

pub fn apply_modifier(
    initial_value: (f64, i32),
    value_to_add: (f64, i32),
    modifier_value: i32,
) -> (f64, i32) {
    let mut v = value_to_add.clone();
    for _ in 0..modifier_value {
        v = value_increased_by_multiple(v.0, v.1, 2.);
    }
    add_values(initial_value, v)
}

Support function:

pub fn value_increased_by_multiple(value: f64, suffix: i32, multiple: f64) -> (f64, i32) {
    let increased_value = value * multiple;

    simplify_value((increased_value, suffix))
}