If you don't feel comfortable using Multiply you might want to communicate with the printer and ask if this is an issue for them to reproduce correctly. Some printers using older rips may have issues with transparency. If you can't speak directly with your printer you could mark up the file or make a PDF to accompany the working file with your comments about what prints at what percentage, and if it should overprint or knockout. Years ago I would get a file with transparency in it and have to make sure the designers intent was clear. This was in some cases due to the designer not understanding blend modes or what he/she wanted overprinting or knocking out in the printed piece. The other way you can confirm the separations are going to rip correctly is to print a PDF out of Indesign and open it in Acrobat pro. Under Advanced>Print Production>Output Preview you can turn on and off separations for the 2 spot colors in question. Because PDF acts as your rip in this case you should be seeing what separations your printer will be printing using your native Indesign file. Some print shops will require you to provide a PDF or a color laser print out for QA of your printed piece when sending files to prepress.
Hope this is helpful.
I would copy the path for the outline paste in front or back. Give the original a stroke of none and leave it as a knock out group.
then give the copy a fill of none, leave the stroke and set it to overprint.
Wade, if he's working in CS3 won't your techneque make the overprint look incorrect in the PDF? I beleive CS3 Acrobat will not show overprints automaticlly as in CS4.
Your techneque is what I would do if I were sending to a printer which has an older rip which didn't support native transpanancy. The latest gen. of rips are using the Adobe PDF rip and can support transparancy through the workflow.