Ehm, jpau, do you really have 18 different primers each of them 20 bp long, or you have just two of them?
Also, you said your primers were 10uM (that means 10umol/L or 10pmol/uL as you calculated). That is a concentration.
You said you need 10pmol of each in the reaction.
That is technically not a concentration, concentration is amount/volume. So, either you need to have a concentration 10pmol/ul in your reaction and then it depends how big is your reaction volume. Or you need 10pmol (which is just an amount) in whatever the final reaction volume may be.
Difference is, for 10pmol in whatever volume you always take the same amount of stock. For final concentration 10pmol/ul in your reaction you need to calculate with the reaction volume (i.e for 20ul reaction you need 200pmol of primer, for 50ul reaction you need 500pmol of primer to get the same concentration).