Hi methuselah,
suggest you will have to moist it at least once, in order to bind the dusty sand/loam-mixture to one (more or less) solid block. Not sure, whether the ants are able to moist it enough themselves so that it won't collapse as they dig chambers and corridors...
Another idea for the setup: How about connecting the basin via "tube fitting T" (
example) between the ant farms?
As young colonies don't need much space, it might happen that the ants use the first farm only for the complete nest (brood & seed chambers) in the beginning...