Yes 'what is ... "normal" ..

My 'one off ' touring frame does not shimmy at all , even with a silly load..

but to achieve that I built it around 2.. 3/4" .049 wall tubes
from a CAT/HPM cargo Bike, Jan helped me put it together..

the tubes are side by side as a top tube,
though they also form the rear triangle as well,
bent at the rear and again at the top of the seat tube.

added bonus the long frame pump sits between the 2 top tubes
so It wont fall off when I hoisted my Bike.

I do have curiosity about whether using a widely ovalized tube 1.125x2.25"?
using thinner wall tube would do the same .. bottom tandem 'Boom Tubes " come to mind..

Tube oriented horizontally for the top and vertically for the downtube..

most factorys use round tube , as its what is most common,
and easier to sell, as people are used to seeing it for 100 years.