One thing that this miserly misfit hermit likes about the idea of joining 2 homes is that,if times get real tough,1/2/side could be closed-off and left unheated/cooled. If I were to do it the unheated side would be the NWerly side as it would make the best winter windbreak.
Around here all mobiles are taxed by floor footage,doubling it would be no major expense,even if unused for a while,or even seasonally. Not sure how they would consider it if permanently joined,so,for that reason,maybe a freestanding roof-over would be cheaper in the longrun.?.?. That way either mobile could be moved after removing the added passages between the 2.
Piers with tie-down bolts poured into them might be a good idea too as running straps over top of a contrived "double-wide" might be an issue with the inspector,,I really dunno that either,,just musing.
PS; An old forman said "If You fail to plan,You plan to fail".