How can I use Go append with two []byte slices or arrays?

The Go Programming Language Specification

Appending to and copying slices

The variadic function append appends zero or more values x to s of
type S, which must be a slice type, and returns the resulting slice,
also of type S. The values x are passed to a parameter of type ...T
where T is the element type of S and the respective parameter passing
rules apply.

append(s S, x ...T) S // T is the element type of S

Passing arguments to ... parameters

If the final argument is assignable to a slice type []T, it may be
passed unchanged as the value for a ...T parameter if the argument is
followed by ....

You need to use []T... for the final argument.

For your example, with the final argument slice type []byte, the argument is followed by ...,

package main

import "fmt"

func main() {
    one := make([]byte, 2)
    two := make([]byte, 2)
    one[0] = 0x00
    one[1] = 0x01
    two[0] = 0x02
    two[1] = 0x03
    fmt.Println(append(one[:], two[:]...))

    three := []byte{0, 1}
    four := []byte{2, 3}
    five := append(three, four...)
    fmt.Println(five)
}

Playground: https://play.golang.org/p/2jjXDc8_SWT

Output:

[0 1 2 3]
[0 1 2 3]