Cooking 2 slabs on the kettle

It is a snug fit depending upon how they are trimmed, but it can be done. I set up a wall in my kettle for smoking using fire bricks that takes up about 1/3 of the space. I pile them as deep as I can with chunks of wood buried so that the cooking grate fits right on top of them. Some people will start start at one end but I just dig a hole in the middle for my lit coals. I put the vent on the opposite end from the coals over the top of the ribs to get it to draw smoke across. Orient the ribs in opposite directions from each other and rotate them to help with even cooking. I also use a foil pan under them. A little water in the pan is optional.

You could create a ring of fire setup. Roll the ribs and secure with a skewer, placing them in the middle of the grill on their bone-end side. That should keep them pretty clear of danger.

Otherwise, you could pile coals on one side and start a small number of coals on top or to one side of the pile. Put an aluminum roasting pan on the other side of the charcoal grate and keep the ribs on that side. You may have to add coals after a bit, but it should work fine as well. Rolling them would also be a good idea to keep them compact.

St. Louis cut spares can be rolled.

Untrimmed spareribs cannot. If you do not have a rib rack, overlapping one slab on top of the other is OK if you rearrange the slabs half way through the cook.