I think that Itzjbean's post is a perfect explanation of how to properly introduce two birds to each other...
You want to do at least a 30-day quarantine with your new bird in a totally separate room that Sammy never has to go into, and if possible behind a latching door...Don't be surprised if Sammy starts acting very weird during the quarantine-period, because he's going to obviously know that there is another bird in the house, and they often display some very odd behavior, often with "attitude"...
What you need to keep in-mind the entire time you're going through this is that you've had Sammy as an only-bird for what, 5 years? So this is going to be very stressful for Sammy, while for the new CAG isn't going to have ANY jealousy, territoriality, dominance, etc. going on at all, and is just going to be a happy, loving, goofy little baby bird that will not know why Sammy doesn't like him (I'm assuming you're bringing home a newly-weaned baby since you're waiting for a certain time to get the bird). So it's going to be extremely important that you put aside some extra time every day for Sammy during the first few weeks/months, depending on how he handles a new bird being added to his flock...
I think sometimes when we bring home a new baby parrot we get so wrapped-up in our new baby that we forget about how effected our current flock-members are by the new addition. I'm totally guilty of this, when I brought Kane (Senegal) home he was only 12 weeks old and he was an adorable little baby bird that actually regressed a little and was crying at night for comfort feedings...So I got all wrapped-up in hand-feeding my new baby his nightly formula, and having Kane on me all the time to make him comfortable, as he was handled all day long by his breeder, so he was used to being with/on someone all day, and I just got all caught-up in my new baby...And the rest of my flock was not amused AT ALL with Kane...first of all he was larger than all of them as a newly-weaned baby, so that was intimidating for them (which will be the case with your new baby CAG as well, he's going to be much larger than an adult Lesser-Jardine)...And I think poor Bowie took it the worst, as he actually started making this new crying noise that he'd never made before, and he only did it at night before bedtime...My CAV told me that his guess was that Bowie was trying to mimic the crying at night that Kane was making because he wanted a comfort-feeding! So poor Bowie was trying to get my attention by crying like the new bird was crying at the same time of day the new bird was crying! I felt awful once I figured out what was going on, and from that point forward I made equal time for all of them, one-on-one...So just keep that in-mind...
After the quarantine is over, I would do exactly what Itzjbean described doing, putting their cages in the same area but not right next to each other, rather across the room from each other so they can see each other and talk to each other, but not so close they are intimidated or feel like their "territory" is being invaded...Again, Sammy may or may not express a lot of territoriality over his cage, his play stands, and his entire "home" in-general, so you have to be very careful that the new baby CAG doesn't get too close to Sammy's "area" until you figure-out what type of relationship they are going to have...
After they have been living in the same room in-sight of each other for a good amount of time, and they've both been out of their cages at different times but not yet together, then you can try to let them out together for the first time, but ON NEUTRAL TERRITORY, AND NOT ANYWHERE NEAR ANYTHING THAT BELONGS TO SAMMY...And no where near the CAG's cage either...It needs to be totally "Neutral" territory that neither bird is going to care about, so whatever part of your home that neither bird spends any time in and has no toys, no stands, nothing of theirs in...And just use your best judgement...
Keep in-mind that this may or may not work-out, meaning they may absolutely love each other and bond closely, they may like each other but not to the point that they snuggle together or share food, etc., they may not really like each other but they will simply "tolerate" each other as flockmates (most common situation that happens), or they may not like each other at all and fight/show aggression towards each other when they get close to one another but still be able to be out at the same time as long as their individual "territories" are kept across the room from each other, or they may absolutely hate each other and actually be purposely-aggressive and go after each other, in which case they can't be out of their cages in the same room at any time, even under supervision...
***You just have to wait and see what type of relationship they have, and then you have to respect that relationship and not ever try to force them to have a different one, because not only does that not work at all, but whenever people attempt to "make" their birds get along and be out of their cages with each other when they know they show aggression 100% of the time, it usually ends in some kind of severe injury of one or both of the birds or of a person, or worse, a tragedy...So it's all about SLOWLY allowing them to get to know each other from "afar", across the room from each other, giving each of them their own "territory", and then slowly giving them more intimate contact to see how they are going to get along...
And keep in-mind that the Poicephalus parrots are all stubborn by nature, lol, and Sammy is probably going to be the one who is going to have an issue with the new bird joining the flock. Sammy was there first and had you all to himself for years, had his territory within your home to himself for years, and suddenly this new, big bird is going to show-up out of nowhere and take away a lot of his time with you...That's how Sammy is most-likely going to feel about this, that's how he's going to see it. Now maybe you'll be incredibly lucky and Sammy will the kind of bird that is really happy to have another bird added to the flock, and he'll accept the new CAG and love him to death...But that's probably not the way it's going to go....That's the point, you just aren't going to know until the new baby comes home. Just try to do your best to not become enthralled with the new baby CAG to the point that you end-up accidentally neglecting or ignoring Sammy without even realizing that you're doing it. it's not hard for that to happen, and usually you won't even be aware that you're doing it because you're so excited about the new baby CAG...Just try your best to let Sammy know that nothing is going to change as far as the flock-dynamic between him and you...