Always a possibility won't like each other, the younger one may try and defend itself if it views the older bird as a threat. The older bird may be terrorial. Higher chance of agression if they are both females. But if your birds are anything like my little lovebird, she loves making new lovebird friends. Do you have a 2nd cage for the new bird? If quarantine is not on your mind yet, I strongly recommend to quarantine the new bird, if you have done disease testing then maybe quarantinerules can be more relaxed?
In general, you should quarantine new birds but I unfortunately have had first hand experience buying lovebirds that have tested positive for transmitable diseases... 3 times in a row. Luckily I had them all tested before bringing them home where I already had birds.