If the router has a USB port (some do), see if it supports printing. If so, plug your printer into there, and it should be pretty easy.
If not, then plug the printer into one Mac, go to System Preferences, open the Print & Fax panel. Check the box next to 'Share my printer with other computers.' Go to second Mac, print a document, and in the printer dialog select the printer that just showed up under the Rendevouz sublist. Done.
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If not, then plug the printer into one Mac, go to System Preferences, open the Print & Fax panel. Check the box next to 'Share my printer with other computers.' Go to second Mac, print a document, and in the printer dialog select the printer that just showed up under the Rendevouz sublist. Done.