Here is a starting place for looking at this- http://www.thingiverse.com/thing:16523
He is using two Makergear hotends with Bowden tubes, which is pretty much what the Solidoodle would do. It also requires a RAMPS. I think trying to force the Sanguinololu to support two extruders is a losing proposition. There will be enough complications without adding that in as well, so it's best to start with electronics that already support the mod. It might be tricky to do dual extruders on RAMPS and still keep a gcode controlled nozzle fan, so the Azteeg X3 would be best I think.
The Azteeg is just over $100 if you don't buy stepper drivers for $50 more. You could keep the Pololu drivers from the Sanguinololu and buy one more for the extra extruder.
KISSlicer is probably the best one to use for dual extrusion. It was developed by a BfB user, and those printers have had dual extruders for a long time. It has separate retraction settings for the unused extruder, so you can pull the filament further back to prevent ooze, and there are also cool down and warmup controls. There is also a wipe pillar- I haven't looked at it much but I believe it is a cylinder that gets built along with the part. When the second extruder is ready to print, it runs over the pillar so that any ooze gets wiped off there rather than on the part.