Warren, the 2 to 1 adapter is machined in two pieces, and comes apart to put the two piece seal inside of the adapter top and bottom.
Then you bolt the adapter together around the two piece crank rear journal with included allen head bolts.
Then set the entire assembly into the one piece block and bolt you other main caps in.
Then bolt the adapter in place just like the one piece flange cover would normally bolt in.
Easy and clever way to retro cross your old crank into a new block.
Jim