We are going to do the 4 stalk OBC retrofit on Alberto's 1997. It does not take any longer to do 2 cars at the same time. In fact, I once did 6 OBC retrofits at the same time. In the picture you see 4 cars in the driveway. There are another 2 cars in the garages.
I have a PST2 so I can turn on the OBC.