The idea from a cluster is normally load balancing. So if you wish that Server 1 and Server 2 handle both the outgoing emails from the users, then you need to split the workload on both servers. BUT that means also that both servers will accept users on it. That means you might end up with more traffic on the leased line! However for the config above you should have a look on the following notes.ini:
Server_Availability_Threshold - By setting this value for each server in a cluster, you determine how the workload is distributed among cluster members.
However this required some ... say advanced tweaking ... as the value must be set according to the server hardware. That means if server 1 is much more powerful then server 2 you might end up with different values for both servers to produce a +/- 50 workload split.
If you use Server_Availability_Threshold=0 on server2 and Server_Availability_Threshold=100 on server1 you could build a kind of failover environment. However some of your applications might not work with that system when they use hardcoded application names! So please be carefully with that config!
Another idea to speed up the routing would be to use multiple mail.box files. You could configure that in the server config (not in the notes.ini). I personally have never used more then 3 mail.boxes on a server. Most servers should be fine with 2 mail.box files.
Another idea to speed up the router process is to fine tune it. For example you could change the MailMaxMessageQueue.