Direct from the wiki.

"You can add these in series to get other values. RT = R1 + R2 + R3 etc...

You can put resistors in parallel to get other values.

For 2 values: RT = (R1 X R2 )/ (R1 + R2)

For 2 or more: RT = 1 / ( 1/R1 + 1/R2 + 1/RN ...)"

So if you are using 2, the product of both values divided by the sum of the values is what resistance you should have.

As far as series, from my knowledge just add the values together to get the total resistance.

-Jonathan