Your sources could be from two pickups (e.g., each humbucker on a Les Paul-type guitar). If both pickups are humbuckers you would need both sections of the blender to have a resistance of 500K Ohms. If you're going to use this with single-coils, use 250K pots. Radio Shack used to carry these in-store and almost certainly has them available through their site.
Or you could mix the total magnetic pickup output with the output from a piezo element. This is a little more difficult and requires a preamp to buffer the signals. The retailer Guitar Electronics has several sample wiring diagrams on their site as to how this might be achieved through one of their preamps.
You could also include a blender on a Strat-style guitar and continue to use the 5-way selector. This would allow you to work with all 5 combinations on a stock instrument, but you could have some variability within those positions which are combinations of two pickups. For example, blend the bridge pickup into the output while you have the neck and/or middle pickups selected.
Simply follow the wiring diagram for a standard pot. Just wire top and bottom stages opposite one another (i.e., reverse "in" and "out" on the other stage).
I should note that I actually don't find this hack particularly useful unless you're blending with piezo pickups. For switching between magnetic pickups, a switch is almost always the better way to go since you probably want the full output most of the time anyway. I don't have a blender on any of my guitars, even the one with a piezo. It's easier (both to wire and to operate) to just have dedicated volume knobs and something to toggle them.