I'm quite certain jmac means to reduce to the volume of the lemon juice, i.e., in your case, from 1-1/2 c to 1/2 c. Personally, I'd use concentrate rather than a reduction, as I think the flavor is better, but I suppose that's a point over which reasonable cooks could differ.
ETA: I almost said the first time that orange and lemon makes more sense to me than orange and lime, but decided that wasn't the question asked. Looking at the canonical lemon curd thread I notice a similar recommendation from another source, so I'll throw it out there.
