I could find a solution.
I used this project: https://github.com/mitre/HTTP-Proxy-Servlet
With that, I could redirect the requests to the other server. In the client view, my own server is answering the request.
In the web.xml, I put the following:
<servlet>
<servlet-name>otherServer</servlet-name>
<servlet-class>org.mitre.dsmiley.httpproxy.URITemplateProxyServlet</servlet-class>
<init-param>
<param-name>targetUri</param-name>
<param-value>http://{_ipOtherServer}:{_portOtherServer}/myAction</param-value>
</init-param>
<init-param>
<param-name>log</param-name>
<param-value>true</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>otherServer</servlet-name>
<url-pattern>/otherServer/action/*</url-pattern>
</servlet-mapping>
Also, in struts.xml I had to exclude all the requests that matched /otherServer.
<constant name="struts.action.excludePattern" value="/otherServer/.*"/>