Recently we discussed the intercropping of coffee and bananas, though this is far from the only possible combination. Here are a few links to other examples of polyculture:
According to the first link the combination of rubber and lemons were never profitable. This because of the low price of lemons. However, this might (or not) be different if the peels would be used to produce textiles.
The second article states that not only does the combination of coffee and macadamia result in much larger yields but also improves soil fertility.