What is the correct way to connect two adjacent panels?
Exterior panels are not normally hermetically sealed, and other than a top entry, no major attempt to seal them is required.
For the meter bottom entry, two locknuts (one inside the panel, one outside the panel) should be sufficient. You actually don't want the bottom of the box to be watertight - any water that gets in or condenses should be able to drain out, though in most boxes it can do that via the open side with the cover anyway.
Same for your question about side to side panels indoors - you don't just leave the conduit floating between locknuts in different panels - or you use one of the cast nipples (with no offset) or offsets that do have a shoulder for the locknut to press against. I would probably use the cast option (offset or not) for the exterior case, but I would not kid myself that it was waterproof, or that it needed to be, but it would keep the size of any water entry from rain hitting it and running sideways down to a minimum.