Place existing BIG-IP's into active/active configuration

It sounds like you have two standalone systems that you want to make into a pair of systems running active/active. First off I would suggest you very carefully look at your traffic and make sure that each BigIP can handle the full traffic of your site without relying on the other.

You are going to need to create the VLANs and relevant non floating self IPs for both traffic groups on each BigIP. Note that you must use different self-IPs on each unit. Once this is done, you will need to pick a single BigIP as the master unit for this process. You will then need to create all the floating configuration (floating self IPs, Virtual Servers, pool members, custom monitors, clientSSL profiles, etc) on this unit.

Once you have the entire configuration on the master, you can add the secondary unit to the HA cluster and sync from the master to the secondary. It will still overwrite the shared configuration, but in this case the configuration will all be present on the master, so it won't matter.

The big caveat in this is that there will be a period where you have virtual addresses for one of the traffic groups on both BigIPs. This will cause address conflicts and service interruptions. You should plan accordingly. Also, make sure you take UCS archives before you start changing anything.

So the article you mentioned is good, but it is assuming that you are starting with new BigIPs. Once you realize that the shared configuration on the master BigIP is going to overwrite the secondary, and that you need to plan for a service interruption things should go fairly smoothly.