Apache can talk to Tomcat using either mod_jk or by using the standard proxy module, mod_proxy. Using the standard proxy module, it's very easy to put multiple instances of Tomcat behind a single Apache instance.
Assuming that you have a Tomcat instance listening on port 8080 and another on port 8081, you can do something as simple as this:
<Location /app1/>
ProxyPass http://localhost:8080/
ProxyPassReverse http://localhost:8080/
</Location>
<Location /app2/>
ProxyPass http://localhost:8081/
ProxyPassReverse http://localhost:8081/
</Location>
This places the first instance at /app1/ and the second instance at
/app2/.
The mod_proxy documentation is a good place to start, and the tomcat documentation covers this topic briefly.