Golang conditional compilation

I've got a trouble with conditional compilation in Go 1.

Here is my test code. Is there anything I misunderstand about the "// +build" constraint and the "-tags" flag?

main1.go

// +build main1
package main

import (
    "fmt"
)

func main() {
    fmt.Println("This is main 1")
}

main2.go

// +build main2
package main

import (
    "fmt"
)

func main() {
    fmt.Println("This is main 2")
}

when running "go build", I still got compile error

$ go build -tags 'main1'
# test
./main2.go:8: main redeclared in this block
        previous declaration at ./main1.go:8