Pesquisando sobre clusterização, acabei encontrando o TerraCotta (http://www.terracotta.org/)

Para executá-lo no modo mais simples, sem fazer uma configuração mais fina, bastou instalá-lo no servidor (terracotta-3.2.1_2-installer.jar) e colocá-lo para funcionar.

Configurei um arquivo (tc-config.xml) e coloquei junto com o start-tc-server.bat – na pasta bin.

C:\Arquivos de programas\terracotta\terracotta-3.2.1_2\bin>start-tc-server.bat -n Server1

O meu arquivo de configuração:

<?xml version="1.0" encoding="UTF-8"?>
<!-- All content copyright Terracotta, Inc., unless otherwise indicated.      All rights reserved. -->
<tc:tc-config xsi:schemaLocation="http://www.terracotta.org/schema/terracotta-5.xsd"
xmlns:tc="http://www.terracotta.org/config"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

<servers>
<!– Sets where the Terracotta server can be found. Replace the value of          host with the server’s IP address. –>
<server host=”10.2.0.101″ name=”Server1″>
<data>%(user.home)/terracotta/server-data</data>
<logs>%(user.home)/terracotta/server-logs</logs>
</server>

<!– If using more than one server, add an <ha> section. –>
<ha>
<mode>networked-active-passive</mode>
<networked-active-passive>
<election-time>5</election-time>
</networked-active-passive>
</ha>
</servers>
<!– Sets where the generated client logs are saved on clients. –>
<clients>
<logs>%(user.home)/terracotta/client-logs</logs>
</clients>
</tc:tc-config>

E nas aplicações, eu coloquei no context.xml (no META-INF) ou diretamente na pasta conf do Tomcat:

<Context>
<Valve className="org.terracotta.session.TerracottaTomcat60xSessionValve" tcConfigUrl="10.2.0.101:9510"/>
<param-value>10.2.0.101:9510</param-value>
</Context>


Pronto, acione o Developer Console e veja os clientes de seu cluster.

Tive alguns problemas para conseguir incluir o ICEfaces (1.8.1) na minha aplicação JSF 1.2 (com MyFaces) + Facelets + Hibernate.

1o passo: incluir os jars necessários
Como já estava utilizando o MyFaces, utilizei o jar just-ice.jar ao invés do icefaces.jar.
backport-util-concurrent.jar
commons-beanutils.jar
commons-collections.jar
commons-digester.jar
commons-fileupload.jar
commons-logging.jar
FastInfoset.jar
icefaces-comps.jar
just-ice.jar
jxl.jar
krysalis-jCharts-1.0.0-alpha-1.jar
xercesImpl.jar
xml-apis.jar

E para utilizar o facelets também no icefaces:
icefaces-facelets.jar

2o passo: configurar web.xml

O Eclipse adicionou essa parte para mim:

<context-param>
<description>To allow multiple windows for a single application.</description>
<param-name>com.icesoft.faces.concurrentDOMViews</param-name>
<param-value>true</param-value>
</context-param>
<context-param>
<description>Turn on/off application-wide synchronous or asynchronous updates.    </description>
<param-name>com.icesoft.faces.synchronousUpdate</param-name>
<param-value>false</param-value>
</context-param>
<context-param>
<description>Google Maps API key is required if gMap component is used. Sign up for an API key from http://code.google.com/apis/maps/signup.html</description>
<param-name>com.icesoft.faces.gmapKey</param-name>
<param-value>ABQIAAAADlu0ZiSTam64EKaCQr9eTRTOTuQNzJNXRlYRLknj4cQ89tFfpxTEqxQnVWL4k55OPICgF5_SOZE06A</param-value>
</context-param>
<context-param>
<param-name>com.icesoft.faces.uploadDirectory</param-name>
<param-value>upload</param-value>
</context-param>
<context-param>
<param-name>com.icesoft.faces.uploadMaxFileSize</param-name>
<param-value>404857600</param-value>
</context-param>

<listener>
<listener-class>com.icesoft.faces.util.event.servlet.ContextEventRepeater</listener-class>
</listener>

<servlet>
<servlet-name>Persistent Faces Servlet</servlet-name>
<servlet-class>com.icesoft.faces.webapp.xmlhttp.PersistentFacesServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet>
<servlet-name>Blocking Servlet</servlet-name>
<servlet-class>com.icesoft.faces.webapp.xmlhttp.BlockingServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet>
<servlet-name>uploadServlet</servlet-name>
<servlet-class>com.icesoft.faces.component.inputfile.FileUploadServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>

<servlet-mapping>
<servlet-name>Persistent Faces Servlet</servlet-name>
<url-pattern>*.iface</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>Persistent Faces Servlet</servlet-name>
<url-pattern>/xmlhttp/*</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>Blocking Servlet</servlet-name>
<url-pattern>/block/*</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>uploadServlet</servlet-name>
<url-pattern>/uploadHtml</url-pattern>
</servlet-mapping>

E essa parte, eu adicionei para que em algumas páginas eu utilizasse o icefaces e em sem.

<context-param>
<description>
The following parameter will cause the ICEfaces ViewHandler
to process only those pages with a .iface extension:
</description>
<param-name>com.icesoft.faces.delegateNonIface</param-name>
<param-value>true</param-value>
</context-param>

3o passo: configurar faces-config.xml
Tive que basicamente mudar as navegações, onde estava pagina.xhtml, mudei para pagina.iface

4o passo:
Testar

Regras:
Nas expressões, se o icefaces estende o html ou o tomahawk, tive que utilizar a implementação do icefaces. Exemplo: onde era mudei para . Podem até coexistir, porém não funcionará corretamente.

Algumas referências:
Icefaces + Tomahawk – Status of ICEfaces Support for MyFaces Tomahawk Components
http://support.icesoft.com/jive/servlet/KbServlet/download/731-102-1045/ICEfacesTomahawkCompSupport.html
PDF – Guia do desenvolvedor do ICEfaces (ver pasta docs do download do ICEfaces)