Client-Server, Threads, client error when going online

I'm developing a cliente-servidor application, the server has two threads (a in and a out ) well implemented.

I run the server (okay), I run the first client and put it as online (okay), I run the second client and when I load it online it gets blocked.

I tried to cycle:

for(int i = 0; i < 2; i++)

I noticed with this cycle that the first client also blocks when going online, that is, the error of my project is here! Can someone help me?

Follow the code:

public PainelEstadoContacto(JanelaUtilizador janelaUtilizador) {
    this.janelaUtilizador=janelaUtilizador;

    botaoOnline= new JButton("ONLINE");
    botaoOnline.addActionListener(e -> {
        String nome = nomeUtilizador.getText();

        if(!nome.isEmpty()){
            this.mensagem = new Mensagem();
            this.mensagem.setAccao(Accao.ONLINE);
            this.mensagem.setNomeClienteEnviaMensagem(nome);

            this.socketCliente = new SocketCliente();
            this.socket = this.socketCliente.estabelecerLigacao();          

            new Thread(new ThreadCliente(this.socket)).start();                              }
        this.socketCliente.enviar(mensagem);

    });