Add avocado and beans to bowl with the salt, garlic, and lime juice. Mash with potato masher until smooth. Add rest of ingredients to bowl, and mix.
This ended up tasting like "avocado light"--it tastes more like avocado than it does beans, and those white beans are pretty innocuous, flavor wise, to begin with. So you get all the avocado flavor, plus more protein and fiber, and you make a whole lot more guac than if you just used one avocado!
Source of recipe: I just came up with this recipe because I wanted some guacamole but wanted something a little lighter on the calories and fat (I know avocados are the good fat, but there's also such a thing as too much of a good thing). I also wanted to stretch my avocado and make as much guac as possible. I just made this up now!