It doesn't haven't to be Multi-master but I do what the two instances to have the same data and be able to update requests to modify/add/delete entries without a refferal..
Lots of FAQ and list archive on this. A syncrepl slave won't allow writes, so that's out of the picture.
If you really need to load balance OpenLDAP 2.3, I submit that something is wrong with what you're doing with the directory -- on decent hardware, you could easily serve tens of thousands of requests/sec.
If all you're really trying to accomplish is HA, there are valid ways of doing it.
John