i am going to purchase the 3.0 update( pioneer). my question is: i have 2 units, will i have to purchase 2 cards or can i use 1 on both?

You need a key number generated from the Pioneer web site, it's one key number per purchase tied to the unit number, so using the same key on a 2nd unit won't work.

It is not a "buy one update, use it on as many machines as you want" situation. It's one update updates one unit.