bro u need the 3rd card for reach level 2 (2 stars)..
watch the stats for each card, on top of it theres exp (above the att stats), it shows u how much card u need for next level..
for example :
exp 1/2 (this is 1 star card)
it means u have 2 card of this liege, and u need 1 more to up level to level 2