What do you mean by "love"? If you mean the way it is often portrayed in media and talked about by people, a feeling of affection, then that may never be the same for your new grand-daughter, or it might take a while, and that is natural. Feelings come and go, they can be affected by something as trivial as weather. A bad cold can make us feel so irritated at everyone in general that we treat a person that we otherwise "love" with the very opposite of affection. Affection is a terrible way to define love, and yet it is what most people mean by it when they say the word.
If on the other hand, by "love" you mean a commitment to put the other person's well-being ahead of your own, for the rest of your life, then you should start to love your new grand-daughter, the same as the other, starting yesterday. If we mean by "love" the kind of patience, kindness and favor that a grand-parent is expected to show their grandchild, then those are actions and attitudes and decisions, not feelings. You can decide to treat both of them the same, no favorites, starting today. Nothing will do more to grow your affection toward your new grand-daughter than a decision of your will that she is the same to you as the other, and that you will treat her so.
Your son should not have had to tell you that you should only send a present to one if you are sending a present to the other. Maybe only your earlier grand-daughter got on the honor roll. If so, it is probably best to avoid the present anyway, just to avoid even the appearance of favoritism.
Real love does not take time to grow. I have loved my children since the moment they were conceived. By that I mean: I have sought the best for them, I have busted my butt making sure they were taken care of, provided for, protected, well trained. That has at times meant 14 hour workdays. Ok, so I worked 14 hours, spent 6 with them (and my wife), helping them with their homework and hearing about their day and praying with them. That left 4 for sleep. Fine, I'll sacrifice my sleep before I sacrifice their well-being. Earlier this year, I had some savings and needed a car. I could have bought a nice new one. Or I could put a chunk on my oldest daughter's account, for her college expenses, avoid her having a bigger debt when she gets out. That meant I would get an older, used car. Not a hard choice to make. You love people with that definition of "love", and the emotions will follow most of the time and the rest of the time emotions won't matter.
I would suggest that you make a trip to see them, if you can, and plan with your son that while you are there, you will take your grand-daughters out on dates, one at a time, your new grand-daughter first. Or send them flowers, the new grand-daughter first, then a couple of days later, your other grand-daughter. Make sure the florist gets the exact same arrangement, and have them sign it "to my beautiful, special princess, from your loving grandma". Commit to your son that you will always treat them the same, and apologize for the error in calling one your "favorite".
You know how special your first grand-daughter has been to you.... you have just had that blessing doubled. It may not feel like it yet, but you can get excited about that, about sharing the awesome grandma-princess relationship with not one, but two special girls. This will not diminish, but multiply that joy for you and for them. Don't worry about the feelings, they will come. But don't waste time waiting for them.