I have done a lot of combining, especially when a particular variety does not germinate well and gives me bare spots. This past few months I have tried the EB Jr size, and they are particularly good for combinations--easier to handle and move around, and useful for the more shallow rooted vegs. There is no particular pattern to what I have combined--I just filled holes with what I wanted to grow.
My EBs are all inside a small heated greenhouse in Colorado. Right now I have the following combinations doing well: In EB Jrs--Beets and leaf lettuce, leeks and leaf lettuce. In regular size EBs: spinach and pole beans, spinach and snow peas, lettuce and bunching onions, zucchini and yellow squash.
TJ