Maybe I'm the last one to know about this, but....
Today was a rare occurrence where I needed to do a search in sixbit. I needed to pull in items where one custom field was larger than another. Of course, in the advanced search there is no such option. However, if you type the database column name in the box where you would normally type the value, it does work.
In my example, I wanted to see pull in items in a certain folder where my custom field ActStock was larger than another custom field, pack.
I built the advanced search, selecting the ActStock Custom field is greater than, and typed Items.CF_pack in the value. Did exactly what I needed.
I would imagine this would work with any field, as long as the joins in the generated query have both tables in it. In my case everything was in items. Haven't tried to compare fields in different tables, and probably will never have the need.