I think having different sized bars is a bunch of crap.
Once they start even the littlest deviation off center, the best way to draw additional comb is placing empty bars in the center of the already made comb area. And then the question starts becoming....."should I rotate in one size or the other?"
Any foundationless system potentially has the problem of curved and messed up comb, if your not using side guides, etc. When I do foundationless systems in regular hives, trench, or other styles, the bees are using standard top bars from frames. The bees have no requirement to make different sized comb.
This whole thing about different top bar widths just makes it harder for no reason. If the bars were all the same, (smaller sized) the bees would do just fine. They store honey in smaller width comb, just as well as wider sizes.
My TBHs never come out that all the brood is on the smaller bars, and all the honey is on the larger width bars. So why try to dictate to the bees?
This is just something someone thought of, then the groupies all started following. Wait....don't take my word for it. Work a season and actually try to manage the bars as to what you think should be used where in the hive. I bet next year, you be thinking different about two sized bars. When they come out of the first winter, and the cluster is not on the designated smaller brood bars, or they fill in the brood area and start raising brood in the larger bars....you will stand there and ask yourself which bars should i add at the end of the comb area. Eventually you will question when the bars are all getting screwed up..."who ever thought of this two bar crap to begin with?"
And why do people still push it?
Two sized bars....absolutely a waste. And not needed. Just as it's not needed or pushed in any other hive we beekeepers make and use.