I am just trying to help here. the only double groove water pump pulleys I have seen are on AC engines. they are much smaller in diameter than the single groove. the only double groove crank pulleys I have seen have a smaller inside groove and the outer is for power steering. a picture of what you are trying to do would help. I have a few left over pulleys from AC and non-ac engines (small block) . the AC crank pulleys have the same diameter grooves, but come in the 3 and 4 groove variety. I may be able to help, but I need pictures.
you can use an ac water pump pulley with two grooves, but it is going to spin the pump faster and may look awkward.
you could also just replace the alternator pulley with a single groove.
I will see what I have and post some pictures