The trick is to roll your finger.

For example:
if the arp looks like this:
-------13---
----12------
-12---------

You would use the tip of your 1st finger to play the 12, then roll your finger so the tip stops sounding the 12 and the part right above the first joint of your finger (see below) hits the 12 on the next string.


-O O
|---| <--- right here
|---|
|---|
|---|
|---|
|---|
|---|

If thats confusing to understand, someone else please help explain. :o