Anna, I would be interesting to know what you think of the relationship between double kettlebell work and starting barbell training.
I'll start by saying I love them both! My own path was: single kettlebells --> double kettlebells --> barbells. I'm a part-time trainer, and I've taken a dozen or so students down a path similar to mine, but I've also taken a dozen or more to barbells first, and several of these from barbells --> kettlebells. There are two other personal trainers that work on my military base, and fortunately they take all the clients that want a "pin setter / reps and sets counter" or "circuit trainer, "dig deep, come on you can do it!" type personal trainer... so I don't have to do that! When a client wants to "get stronger" and/or specifically get instruction and coaching on barbells or kettlebells, they ask for me.
I am just wondering when people can get the most bang for their buck in a GPP general population.
Either one is viable to start with, and can be coached and regressed as needed for any trainee. "Which is better to start with" depends on a lot of factors, but primarily -- 1) whether they have a particular desire to do one or the other, and 2) how quickly they want to get strength results. If their aim is to get strong and they want it to happen fast, we go straight to barbell. If they want strength as part of a general improvement (along with movement quality, mobility, conditioning, and skill), and they want to be empowered to train on their own after 3 or 10 sessions, or to be able to train outside the gym, we go with kettlebells.
It takes a while to get to double kettlebells. It varies by trainee, but in general at least 10 sessions and/or 2 months. Even more is probably better. I usually spend 1-3 sessions with the kettlebell deadlift and hinge pattern plus 1H presses by using a "cheat clean" (2-handed slow clean), then 3-5 sessions with 2H swings and get-ups, then if that's going well and they're getting more practice on their own and actually progressing their strength and skill with the 2H swing, we can start 1H swings and then double swings. Then the proper single and double clean, which opens up the options for double kettlebell front squat and double press.
In contrast, I can get someone pressing the barbell overhead in the FIRST SESSION, and moving more than double the weight than what they can do with the 1H kettlebell press... also deadlifting more than they can KBDL, and squatting way more than they could goblet squat (and we can't double kettlebell front squat for a while, as I described). In the second session we increase the weight on the bar and do the bench press instead of the press. In the third session, we increase the weight again and alternate back to press. Etc. ALWAYS coaching good movement. OCCASIONALLY regressing the movement to a smaller range of motion (i.e. elevating the barbell for deadlifts, or working down to full depth for squats) -- but I have not found this to be necessary for many people at all. Most just need to be coached to proper movement.
I was shocked how challenging double KB front squat was.
Yes it is! It sort of forces that upper back tightness that you can find with a "face the wall" squat. LOTS of uncoached barbell squatters are missing this. I coach it relentlessly from the very beginning on barbell squats... credit to what I've learned from Starting Strength on that.
Since barbell work requires more mobility and attention to form than KBs
I wouldn't say that. The both use and improve mobility, and they both require a ton of attention to form. That said, you can do "lighter" work with kettlebells that "allows" you to put less attention on form -- but if you're properly loaded with either one, form becomes paramount.
do you think the barbell press had advantages over just using double KB press to improve your double KB press or could you have just done double KB press
The microloading of the barbell was key for me. Once you get to "pretty strong" on overhead press, it takes a lot of work to add lbs or kgs, and it happens slowly. With the big KB jumps, you have to so some fancy volume programming to progress. With barbell, you can microload and get just exactly the right stimulus.
I got feedback at the barbell course that my thoratic mobility needed work before barbell press and low bar squat.
I couldn't say for sure unless I coached you directly, but I've coached a lot of people with "average" (i.e. poor) thoracic mobility when they start out, and can get them to barbell press and low bar squat. It improves as they do it, and they get stronger as they go.