I agree with Laney and Dennis....If they are both juvi's they might not show aggression but you never know when they will and the new one should be quarantined for at least 90 days. You dont want your current gecko to get anything the new gecko might be harboring.
Yah, as said above by everyone you gotta quarantine!!!!! Diseases and parasites can all occur if you don't quarantine, and as has also been mentioned, if you don't know the sex of the new leo its a problem in itself since they have the potential to fight out of the blue on the off chance that they're both males. And on top of all of THAT, you might not see aggression yet but it could still be there. When we tried introducing a tank-mate to Rin it appeared fine at first but it soon became obvious that the two did not get along. You can't always tell immediately so caution is the best policy!!!