Spermicide, in spermicidally-lubricated condoms, is a chemical called nonoxynol-9, which is essentially dish detergent.
At Scarleteen, we aren't big fans of spermicide for a number of reasons. The World Health Organization has for some time been reconsidering promoting its use, after seeing "an increasing number of complaints by women using spermicides and contraceptive sponges with nonoxynol-9 who experienced vaginal and cervical ulcers, burning sensations, and recurring yeast infections."
Simply because so many people (at least 10% of all people) have high sensitivity or allergic reactions to nonoxynol-9 makes us wary. When your genitals become at all inflamed or sensitive, it in fact makes you MORE susceptible to getting a disease -- even with a condom.
In addition, overall, spermical lubricant (and pre-lubricated condoms) causes more condom wear and tear than simple-water based-lubricants. So, if your condom breaks, you only have the protection of the spermicide (which is usually only at about 80%) in terms of pregnancy, and none in terms of disease.
So, to answer your question, it doesn't double your protection, and may, in fact, make you LESS protected.
The best thing we know of is to use a NONlubricated condom properly using a latex-safe and water-based lubricant with it (like Astroglide, KY Liquid, ID, Wet, and so forth).
You can get extra condom info here: http://www.scarleteen.com/pink/pages/condom.html