I understand that some 2-barrel Carter BBD's were spec'd for the slant 6, which obviously would mean that for some make/model/year(s) the slant 6 came with a 2-barrel intake. (my only experience with a slant 6 was the '65 Canadian Polara I owned and drove from '84 to about '96 and it had single barrel Carter BBS).

I'm wondering about some aftermarket (china-made) BBD carbs that are described as being for the 318 and the slant 6, and I'm wondering how they can be set up correctly (jetted, etc) for both engines.

They can't. There were enough internal differences between the slant six Carter BBD and the 318 Carter BBD that in 1976 Chrysler published a special manual describing the differences between the two. See the last 20 or so pages of the BBD manual available HERE

Further, there are noteable external differences, most importantly the linkage for the choke valve. The 318 BBD uses a different linkage that will not work with the slant six choke thermostat. HERE

A simple solution has been devised for the choke linkage. See HERE

Many people have successfully run a 318 BBD on a slant six, sometimes having to plug the additional holes in the throttle blades, sometimes not.

Once upon a time, it was possible to buy metering rods and jets from Carter to fine tune the BBD. Nowadays you have to be content with jets from older Edelbrock Performer carburetors that may or may not fit the Chinese BBDs.

Aside from those differences, I would be very suspect of any cheap carb coming from China. Who knows what (or if) it is calibrate for. Who knows if the proper and necessary protective coatings are present.

These days, if I were in the market for a two barrel carburetor for a slant six, I would look for a Holley 2300 350 CFM model and use a Mr. Gasket 1937 adapter on top of the factory two barrel intake.