Hi toki,
First of all it depends on how much the company would like to invest in such a methodology. Second of all, why would you make a line to be single lane - double line one after each other, is that automotive / medical standard, not to handle PCBs manually? For that you will need a huge area or turntables. But let's go back to advantages and disadvantages of both.
Dual lane is not necessarily good for assembling both sides at one go. So what will you do until the first side is built? The other lane will stay idle? It's the same vice versa. That's okay as Davandran said, mostly for non-complex board which requires smaller kitting / set-up.
Single lane double line sounds okay, you can use them for other products as well, but you need more space / turntables and brides in-line, so operators don't have to walk for too long if there's a problem on the other side. You can as well build separate products on the two lines as well, and as you wanted, . I'd go for this one, despite it would be a massive investment. And don't forget about PCB flipper if you want to avoid handling manually :)
The other thing I saw, two separate lines to use a common oven and AOI, which worked in a high volume - low mix company, but is not intended to be for assembling both sides at one go (it's pretty obvious)
Kind regards, Tom
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