You create 2 color separations by only using 2 colored inks in your document. Use your separation preview function to see if you have only 2 colors in your document.
Your drop shadow color needs to be one of those 2 inks and you should choose a transparency mode that does not try to pull in other CMYK inks. Again, check this with your separation preview function.
When all looks good, you can export to a PDF file and let the printer extract the 2-ink data using his up-to-date Acrobat software. If you are using a printer that needs you to supply 2 different PDF files for each ink because they do not have reasonably up-to-date Acrobat software, you can print to a PDF file with separations (you must own Acrobat to accomplish this).
- - I hope this helps - -
(Complain to someone else if it doesn't)