Golang: Stack multiple method calls on one line

Getting started with Go. I'm trying to write a function that title cases a first name then caps the second. Why can't I stack method calls as below?

(The reason why I want to put a .ToLower before is because the .Title only caps the first letter leaving the rest unchanged)

package main

import (
  "fmt"
  "strings"
)

func main() {
  firstName := "mElVIn"
  lastName := "themelvINATor"
  fmt.Println(nameCap(firstName, lastName))
}

func nameCap(s1, s2 string) (str1, str2 string) {
  s1 = strings.ToLower(s1).Title(s1)
  s2 = strings.ToUpper(s2)

  retu s1, s2
}