Hello Pedro. This is very easy.
First configure one of the units as a new cluster. During the setup wizard, there will be an option that asks you if the unit is a New Cluster or if you are joining an existing cluster. At this point, the first unit should be new cluster.
Once you enter all network configuration and finish naming your cluster (write down the cluster name and cluster interface IP if doing more than one IP address or port group).
After you are done with the setup and this unit is online and running (you are able to get to the
https://:8443 successfully and login) then you are ready to begin configuration of the other unit.
When you get to the spot of setting up a new cluster or join an existing one, chose join. Enter the existing cluster information and make sure that they are connected to the same switch and they can reach each other via their respective cluster interface.
Once all info is entered, go ahead and click next, confirm a few things and then this new unit will synchronize with the existing unit and download databases and such.
You are now ready to support 1000 APs on an Active/Active cluster, not Active/Standby as we prefer all Active nodes for much more efficiency.
Thanks,