This article is from the the official alt.spleen FAQ, by Andrew Stellman with numerous contributions by others.
Andrew says: NOTE: PLEASE DO NOT SEND ANY MAIL TO ME REQUESTING MEDICAL ADVICE! I AM NOT A DOCTOR, AND WILL NOT GIVE YOU ANY USEFUL INFORMATION.
Record-breaking track star, total genius, and spleen expert Tom Whelan (email@example.com) has this response:
"Actually, a 'second Spleen' is incorrect. Some folks have what is called in the medical domain as an 'accessory' Spleen. This is usually a lobulation connected to the primary in some fashion. It may be fed by a secondary branch of the splenic artery, arising, usually from the celiac axis. Before a person can donate the accessory spleen, it should be noted that this involves major surgery (not to be taken lightly by the faint-of-heart-) and should only be an endeavour involving a person who is an experienced donor. If that person decides to donate the accessory spleen, Have the donor get real sick first...ie get an infection. This will usually cause the Spleen to get very large, thus making it easier to work on, and result in a larger specimen for the collection. I hope you find your donor at a reasonable cost. Consider trading an anatomically neglected portion of your anatomy with the donor...:)"