I am currently training 2 lineolated parakeets (brothers from same clutch) and they are doing--what I would consider--marvelously at this point. I am an amature in this, but I think they are just brilliant! We've been doing 15 minute clicker training sessions 3x a day (which they really seem to enjoy---I am using bird crack (ahem) I mean spray millet as a reward).
I've only had them for a little over a week, and they have both mastered step up, are walking up and down the outside of their big flight cage to target the end of a wooden skewer, are giving kisses on the lips (they just lean forward and touch your lips with their beak), and very reliably waving hello with their foot on command. These are their "strengths."
Now, they are not at the age of maturity yet, so maybe our good luck will wear off some later on as they age and go through bird-puberty. They are about 11 weeks old (fledged around 7 weeks), and were raised by the best breeder anyone could dream of getting a baby from. They came to me very tame, very socialized, and very, very sweet (both already loving scritches and snuggles when sleepy). So this may also be a significant factor in our training success?
But what I wanted to say, which seems interesting considering multiple sources say to train birds individually and not together, including this thread, is that my two seem to benefit from training together. My Doodle actually learned to wave by watching Delta wave and get c/t. I NEVER taught Doodle to wave, but after 1 day of watching he is now waving on command like a pro for a c/t. He totally gets it. I couldn't believe it--I was asking Delta for 'wave' and they both just waved at the same time.

Now how can this be explained? They each wave on command, but I only taught one that trick.
Maybe Linnies learn differently than budgies, I would not know about that. But in our situation, training as a "flock" is working to our benefit at the moment. They are trying so hard for that c/t and are learning from each other how to get more c/t's. I would imagine it would be very "distracting" to separate them during the training (they will call to each other
very loudly if they are not together.)
They will ride together on my shoulder, snuggle up together with a blanket on the couch with me in the evenings, and they both fly to me at random (seems to always be when I am cleaning--they will fly to my head or cling to my shirt--silly birds!) They seem to be bonded together but also are bonding together with me, if that is possible? Again I am new to this. Maybe this will change later on as they reach maturity? I hope not.
Any thoughts? I don't mean to come in here and throw a curveball against the standard advice. I really don't know anything. I am wondering if training 2 birds together has ever worked out well for others like it seems to be working for me and my two. I just saw this thread and felt compelled to share my sucess thus far. I appreciate the experience and knowledge that is here, and would love to hear opinions. Is our course of action going to fail later on down the road?
I am just a newbie to the bird training world. But happy to be here.

Thanks!!

Oh! and I want to also mention how I manage to c/t for 2 birds at the same time. What I am doing, is I will ask something of 1 bird (at this point I ask for step up, step down, target, give kisses, or wave) and then I will click and treat that bird with just a tiny broken off piece of millet (a little cluster about the size of a pea). Then I immediately will ask something of the 2nd bird, while the 1st bird is still chowing down on his treat. I just trade back and fourth, letting one eat his treat while the other is working for a treat. Works good so far. Linnies are not known for fast movements though, they are rather slow-walking and deliberate, so that probably helps us. Also I am making a point to always say the name of the bird I am giving a command to...in case they may eventually learn their names and know who I am talking to. I hold the millet and clicker in one hand, with the target stick in my free hand. Sometimes I hold everything in one hand though, or will put the target stick under my arm at my side.