I'm pretty sure selling in two different categories is not available to all categories anymore, and that computers / electronics is one of them (at a guess, I would say it's primarily because there are significantly reduced fees for electroncs over $200 when the seller has a store, and it probably helps avoid miscategorisation etc to try and take advantage). 

Edited to add - there is nothing on eBay's help page that mentions category restrictions http://pages.ebay.com.au/help/sell/two-categories.html , but I also know you're not the first to find this issue with computers / electronics.