Load balancing ESAs is all in how you get the traffic to them, they don't communicate or monitor one another.
The "easiest" is to put a load balancer cluster in front of them (F5, Citrix Netscaler, etc...), just like for web sites, but on port 25...
You can do some with DNS MX records. you'll get something close to balancing... your DNS provider may allow you to "load balance", where they'll give out different addresses for the same request on a round robin or weighted fashion.
For outbound mail, your email software may be smart and balance which ESA it sends to, or it may not, in which case, a load balancer may be your best bet....
Look at the overview reporting page to see if they are getting similar traffic volumes....
