With latest release of IBM's OmniFind Yahoo Edition, users can generate up to five separate indexes of documents and define additional custom search fields
W. Cavnar, and J. Trenkle. Proceedings of SDAIR-94, 3rd Annual Symposium on Document Analysis and Information Retrieval, page 161--175. Las Vegas, US, (1994)
S. Chakrabarti, M. Joshi, and V. Tawde. Proceedings of the 24th Annual International ACM SIGIR Conference on Research and Development in Information Retrieval, page 208-216. ACM Press, (2001)
Y. Chiaramella. Lectures on Information Retrieval, Third European Summer-School, ESSIR 2000, volume 1980 of Lecture Notes in Computer Science, page 286-309. Springer, (2001)
W. Cooper. Proceedings of the 17th Annual International ACM SIGIR Conference on Research and Development in Information Retrieval, page 242-247. Springer, (1994)