1) We need more info here. Apache what? HTTPD? Tomcat? I'm going to assume Tomcat now what version are you using? 6.0.x, 7.0.x?
2) How do you want to persist your sessions? Memcached is an in-memory cache, so calling it persistence is a stretch. It is a good way to make your sessions available to multiple Tomcat servers though, so you just have to think about exactly what you want here.
Out-of-the-box Tomcat supports persistence of sessions to disk and to a relational database. Reference docs can be found here.
Configuration is pretty simple, you just need to adjust the tag in server.xml, and if using a relational database create the table.
I just want the tomcats to either a) direct all session traffic to a single node
This sounds like you're referring to sticky sessions and that something your load balancer would do, not Tomcat. Also, this has nothing to do with session replication. It just binds a user's requests to one particular Tomcat node and that node happens to contain the data for that session. If the node hosting a particular session went down, you'd lose your user's session data. In some cases that's OK though, it just depends on your needs.
If you need something with better availability, you could pair it with session replication or session persistence.
Here's a link to the docs for session replication. The link for session persistence is above.
or b) make the two tomcats aware of all sessions. Can someone point me in the right direction?
See links above. Try to follow the instructions there.