This is easy way to do it
2 TEEs 3 shut-offs 2 hot water machines and 2 reels. the machines do not go through the same coil. this is after the burner before the reel. close the top one means both machines go to the bottom reel. close the middle one you have two seperate machines and reels. close the bottom one you have both going to the top reel. it costs bout 90 to 100 dollars but saves time on hooking stuff up taking it apart, and all.